Support running setup.py from other directories.

This incorporates changes from pyocd#753 by @jsiverskog.

setup.py

@@ -1,5 +1,5 @@
 # pyOCD debugger
-# Copyright (c) 2012-2019 Arm Limited
+# Copyright (c) 2012-2020 Arm Limited
 # SPDX-License-Identifier: Apache-2.0
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
@@ -19,22 +19,28 @@
 from setuptools import setup, find_packages
 import zipfile
 
+# Get the directory containing this setup.py. Even though full paths are used below, we must
+# chdir in order for setuptools-scm to successfully pick up the version.
+SCRIPT_DIR = os.path.dirname(os.path.realpath(__file__))
+os.chdir(SCRIPT_DIR)
+
 # Read the readme file using UTF-8 encoding.
 open_args = { 'mode': 'r' }
 if sys.version_info[0] > 2:
     # Python 3.x version requires explicitly setting the encoding.
     # Python 2.x version of open() doesn't support the encoding parameter.
     open_args['encoding'] = 'utf-8'
 
-with open('README.md', **open_args) as f:
+readme_path = os.path.join(SCRIPT_DIR, "README.md")
+with open(readme_path, **open_args) as f:
     readme = f.read()
 
 # Build zip of SVD files.
 #
 # The SVD files are stored individually in the data/ directory only in the repo. For both sdist and
 # wheel, the svd_data.zip file is used rather than including the data directory. Thus, this setup
 # script needs to skip building svd_data.zip if the data/ directory is not present.
-svd_dir_path = os.path.join(os.path.dirname(__file__), "pyocd", "debug", "svd")
+svd_dir_path = os.path.join(SCRIPT_DIR, "pyocd", "debug", "svd")
 svd_data_dir_path = os.path.join(svd_dir_path, "data")
 svd_zip_path = os.path.join(svd_dir_path, "svd_data.zip")
 if os.path.exists(svd_data_dir_path):