Minor follow-on to PR #1334 (Fix enum value's __int__ returning non-int when underlying type is bool or of char type) (#3232)
* Minor tweaks.
* Restoring tests/pybind11_tests.h version from master, removing just the comment and empty line that was added in PR #3087; those were made obsolete by the pragma cleanup that concluded with PR #3186.
* More-to-the-point test for Python 3.
