Automatically update Pyhon package description from GitHub repo description

[redeboer]

It would be cool if the GitHub repo description is synced with the package description in setup.cfg (as was introduced by e.g. ComPWA/qrules#102). This can be done with PyGithub:

>>> from github import Github
>>> github = Github()  # no need for access token
>>> repo = github.get_repo("ComPWA/qrules")
>>> repo.description
'Rule-based particle reaction problem solver on a quantum number level'