- add pkg in requirements-dev.txt
- require pkg in pre-commit config
- add some easily resolved annotations in tests
- ignore some type warnings in tox.ini

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

Closes: #856
This change adds the flake8-annotations plugin to enable linting of type annotations via pre-commit/tox. In order to enable that without breaking the build, I've added some annotation warnings to the flake8 ignore lists. I thought it would best be left to the maintainers to decide whether it would be more appropriate to address those warnings in separate commits/PRs.

The ignored warnings, and some brief discussion:

• Globally ignored:
  • ANN101 Missing type annotation for self in method / ANN102 Missing type annotation for cls in classmethod
    • PEP 484 says, "In most cases the first argument of class and instance methods does not need to be annotated, and it is assumed to have the type of the containing class for instance methods, and a type object type corresponding to the containing class object for class methods." flake8-annotations warns on this case anyway; given the large number of changes required to "fix" this warning I thought it better here to ignore them and have a smaller commit that still advanced the state of the project. Of course if they're desired it would be cleaner to address that issue separately from whether to lint for annotations at all.
• Ignored only in the tests dir:
  • ANN201 Missing return type annotation for public function
    • Adding such annotations to all tests would again make the commit pretty large.
  • ANN001 Missing type annotation for function argument
    • This warning appears throughout the tests, including in the pytest fixtures in conftest.py; again, seems like a change worth discussing separately from existing annotations.
  • HOWEVER, I did add a very few annotations in the test suite nonetheless, as they seemed to me not to be very intrusive.