libsystemd-dev needed also for installation via pip

[ilario]

Installing this package with:

pip install systemd-python

On a Debian oldstable (Bullseye), I got an error message very similar to the ones reported in #47 and #97:

pip install systemd-python
Collecting systemd-python
  Downloading systemd-python-235.tar.gz (61 kB)
     |████████████████████████████████| 61 kB 83 kB/s
    ERROR: Command errored out with exit status 1:
     command: /opt/measurement_system-venv/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-3lp1v0wl/systemd-python_605170e387494e1a9bdc2c7ba6ddef4a/setup.py'"'"'; __file__='"'"'/tmp/pip-install-3lp1v0wl/systemd-python_605170e387494e1a9bdc2c7ba6ddef4a/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-zqy_tu5b
         cwd: /tmp/pip-install-3lp1v0wl/systemd-python_605170e387494e1a9bdc2c7ba6ddef4a/
    Complete output (12 lines):
    Cannot find libsystemd or libsystemd-journal:

    Package libsystemd was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd' found

    Package libsystemd-journal was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd-journal.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd-journal' found

    ----------------------------------------
WARNING: Discarding https://files.pythonhosted.org/packages/10/9e/ab4458e00367223bda2dd7ccf0849a72235ee3e29b36dce732685d9b7ad9/systemd-python-235.tar.gz#sha256=4e57f39797fd5d9e2d22b8806a252d7c0106c936039d1e71c8c6b8008e695c0a (from https://pypi.org/simple/systemd-python/). Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
  Downloading systemd-python-234.tar.gz (53 kB)
     |████████████████████████████████| 53 kB 496 kB/s
    ERROR: Command errored out with exit status 1:
     command: /opt/measurement_system-venv/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-3lp1v0wl/systemd-python_fce97db8c3184d04b9266626a33ab67b/setup.py'"'"'; __file__='"'"'/tmp/pip-install-3lp1v0wl/systemd-python_fce97db8c3184d04b9266626a33ab67b/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-2hsisnk0
         cwd: /tmp/pip-install-3lp1v0wl/systemd-python_fce97db8c3184d04b9266626a33ab67b/
    Complete output (12 lines):
    Cannot find libsystemd or libsystemd-journal:

    Package libsystemd was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd' found

    Package libsystemd-journal was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd-journal.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd-journal' found

    ----------------------------------------
WARNING: Discarding https://files.pythonhosted.org/packages/e8/a8/00ba0f605837a8f69523e6c3a4fb14675a6430c163f836540129c50b3aef/systemd-python-234.tar.gz#sha256=fd0e44bf70eadae45aadc292cb0a7eb5b0b6372cd1b391228047d33895db83e7 (from https://pypi.org/simple/systemd-python/). Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
  Downloading systemd-python-233.tar.gz (49 kB)
     |████████████████████████████████| 49 kB 1.6 MB/s
    ERROR: Command errored out with exit status 1:
     command: /opt/measurement_system-venv/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-3lp1v0wl/systemd-python_bb46b5166fe1470496a4586b7b1b3884/setup.py'"'"'; __file__='"'"'/tmp/pip-install-3lp1v0wl/systemd-python_bb46b5166fe1470496a4586b7b1b3884/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-95vgxi6u
         cwd: /tmp/pip-install-3lp1v0wl/systemd-python_bb46b5166fe1470496a4586b7b1b3884/
    Complete output (12 lines):
    Cannot find libsystemd or libsystemd-journal:

    Package libsystemd was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd' found

    Package libsystemd-journal was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd-journal.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd-journal' found

    ----------------------------------------
WARNING: Discarding https://files.pythonhosted.org/packages/10/89/d66ae73bdbc2614e4f2e90ddf9ef80d22c28f3cd70071345c3640465c14c/systemd-python-233.tar.gz#sha256=06a3768b744884565d7961b44f4a51d164765bc59692f0f85ea6f11330a17da4 (from https://pypi.org/simple/systemd-python/). Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
  Downloading systemd-python-232.tar.gz (49 kB)
     |████████████████████████████████| 49 kB 1.4 MB/s
    ERROR: Command errored out with exit status 1:
     command: /opt/measurement_system-venv/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-3lp1v0wl/systemd-python_f10a8673949f4842a0bb53f7ea5fe779/setup.py'"'"'; __file__='"'"'/tmp/pip-install-3lp1v0wl/systemd-python_f10a8673949f4842a0bb53f7ea5fe779/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base /tmp/pip-pip-egg-info-oql0295y
         cwd: /tmp/pip-install-3lp1v0wl/systemd-python_f10a8673949f4842a0bb53f7ea5fe779/
    Complete output (12 lines):
    Cannot find libsystemd or libsystemd-journal:

    Package libsystemd was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd' found

    Package libsystemd-journal was not found in the pkg-config search path.
    Perhaps you should add the directory containing `libsystemd-journal.pc'
    to the PKG_CONFIG_PATH environment variable
    No package 'libsystemd-journal' found

    ----------------------------------------
WARNING: Discarding https://files.pythonhosted.org/packages/87/a3/e73a8373d40b05a75bb34fdccae606ab4dc0c0743535289904f15eb624f5/systemd-python-232.tar.gz#sha256=e5d0fdbdf50e7d67c77e08bc7986ef41d98aef222896bc033c27477893c3d4ba (from https://pypi.org/simple/systemd-python/). Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
ERROR: Could not find a version that satisfies the requirement systemd-python
ERROR: No matching distribution found for systemd-python

Installing libsystemd-dev with apt install libsystemd-dev allowed me to install the package.

I would mention this also in the "Installation" section of the Readme (that is the section I read when I tried to install via pip...) and not only in the "To build from source" one.