Update pygithub requirement from ~=1.54 to ~=1.55

Updates the requirements on [pygithub](https://github.com/pygithub/pygithub) to permit the latest version.
- [Release notes](https://github.com/pygithub/pygithub/releases)
- [Changelog](https://github.com/PyGithub/PyGithub/blob/master/doc/changes.rst)
- [Commits](PyGithub/PyGithub@v1.54...v1.55)

Signed-off-by: dependabot[bot] <support@github.com>

requirements-dev.txt

@@ -2,6 +2,6 @@
 black==20.08b1
 packaging==20.9
 pre-commit~=2.12
-PyGithub~=1.54
+PyGithub~=1.55
 pyupgrade~=2.13
 yamllint~=1.26