Project CI
* Adds `pre-commit` CI by sobolevn in https://github.com/typeddjango/djangorestframework-stubs/pull/360
* Fix strict mypy errors in plugin and scripts by intgr in https://github.com/typeddjango/djangorestframework-stubs/pull/363
* CI: Run mypy on self check by sobolevn in https://github.com/typeddjango/djangorestframework-stubs/pull/361
**Full Changelog**: https://github.com/typeddjango/djangorestframework-stubs/compare/1.9.0...1.9.1