bpo-38914 Do not require email field in setup.py. (GH-17388)

bpo-38914 Do not require email field in setup.py. (GH-17388)

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