BLD: remove generated Cython files from sdist

Rationale: it has happened multiple times in the recent past that users
needed to use a more recent Cython than was used to generate the sdist
of the most recent release available on PyPI (e.g. for building with
a not-yet-released version of Python for which a Cython fix just
landed).  The dependency specification and packaging tools have matured
to the point where it should be fine to require Cython to be installed.
Also, we have wheels (and conda-forge packages) for Windows, Linux and
macOS, so this change won't be visible at all to the average user.

Closes numpygh-13790

Also adds pyproject.toml to MANIFEST.in, it was missing.

MANIFEST.in

@@ -5,6 +5,7 @@
 # Avoid using MANIFEST.in for that.
 #
 include MANIFEST.in
+include pyproject.toml
 include pytest.ini
 include *.txt
 include README.md

pyproject.toml

@@ -1,6 +1,10 @@
 [build-system]
 # Minimum requirements for the build system to execute.
-requires = ["setuptools", "wheel", "cython"]  # PEP 508 specification
+requires = [
+    "setuptools",
+    "wheel",
+    "Cython>=0.29.13",  # Note: keep in sync with tools/cythonize.py
+]
 
 
 [tool.towncrier]

setup.py

@@ -422,8 +422,8 @@ def setup_package():
     if run_build:
         from numpy.distutils.core import setup
         cwd = os.path.abspath(os.path.dirname(__file__))
-        if not os.path.exists(os.path.join(cwd, 'PKG-INFO')):
-            # Generate Cython sources, unless building from source release
+        if not 'sdist' in sys.argv:
+            # Generate Cython sources, unless we're generating an sdist
             generate_cython()
 
         metadata['configuration'] = configuration

tools/cythonize.py

@@ -74,6 +74,7 @@ def process_pyx(fromfile, tofile):
         # Cython 0.29.13 is required for Python 3.8 and there are
         # other fixes in the 0.29 series that are needed even for earlier
         # Python versions.
+        # Note: keep in sync with that in pyproject.toml
         required_version = LooseVersion('0.29.13')
 
         if LooseVersion(cython_version) < required_version: