bpo-33388: Add variables to distutils/dist.py support PEP 566 metadata by rbricheno · Pull Request #6642 · python/cpython
These variables are used in setuptools to support PEP 566 'Description-Content-Type' and 'Provides-Extra':
https://www.python.org/dev/peps/pep-0566/
http://setuptools.readthedocs.io/en/latest/setuptools.html#metadata
Currently dist.py in CPython will warn if they are set, e.g.:
dist.py:261: UserWarning: Unknown distribution option: 'long_description_content_type'
The warnings seem spurious, but looked a bit scary to me as a new user. By declaring these variables, the warnings are no longer displayed. long_description_content_type, at least, is being used in the wild since: