Redirect Python manifest to less ambiguous filename

INVENTORY.md

@@ -11,3 +11,4 @@
 * [setup.cfg](setup.cfg)
 * [setup.py](setup.py)
 * [tox.ini](tox.ini)
+* [PYTHON-MANIFEST.in](PYTHON-MANIFEST.in)

setup.py

@@ -36,6 +36,10 @@ import sys
 from distutils import core as _core
 from distutils import extension as _extension
 import setuptools
+from setuptools.command import egg_info
+
+# Redirect the manifest template from MANIFEST.in to PYTHON-MANIFEST.in.
+egg_info.manifest_maker.template = 'PYTHON-MANIFEST.in'
 
 PYTHON_STEM = './src/python/grpcio/'
 CORE_INCLUDE = ('./include', './',)