Now that GitHub have moved their documentation from
developer.github.com, we should update our links. I have also tried to
update the call strings to their new format, but since it was done via
regex, some of them may not match exactly.
Deployment, TimelineEvent{,Source} were missing mypy stubs, since they
were added after the mypy branch was started. Add them, and clean up two
spurious __future__ imports that are no longer required. Add mypy's
cache directory to .gitignore
Trim21