tox environment: assert on Cython source file presence

Add an assertion somewhere that checks if Cythonized files are present in the sdist tarball in use. That is for "tox -e py27" assert that these files are not present, for "tox -e py27-cython" assert that they are present.

Imported from Jira [BEAM-8572](https://issues.apache.org/jira/browse/BEAM-8572). Original Jira may contain additional context.
Reported by: udim.

Locate the tox definitions for the py27 and py27-cython environments, then trace how the sdist tarball in use is selected or inspected. Add checks matching each environment's expected Cython source-file presence, and verify both tox environments pass with the appropriate assertion.

Written by the indexing model from the issue text.