When checking `setup.py` and when the `author` field was provided, but
the `author_email` field was missing, erroneously a warning message was
displayed that the `author_email` field is required.
The specs do not require the `author_email`field:
https://packaging.python.org/specifications/core-metadata/#author
The same is valid for `maintainer` and `maintainer_email`.
The warning message has been adjusted.
modified: Doc/distutils/examples.rst
modified: Lib/distutils/command/check.py
https://bugs.python.org/issue38914
9f9dac0a4e