Bot(s) to help PyPA's working processes.

Together, @pypa-bot and @Browntruck handle:

• Checking that news file entries are added for PRs to pip
• Commenting and labelling PRs that can't be merged due to merge conflicts
• Dismissing reviews from maintainers based on request from PR authors

NOTE: We're currently midway through porting the functionality to Twisted.

This can be seen on the twisted branch. Some functionality (checking news entries, unlabelling merge conflict PRs) is handled by that branch currently.