To facilitate downstream users who want to use static type hinting in their codebase, it seems like it's a good idea to use the mypy-style static type hinting in the dateutil codebase. Some of this work has already been done in the typeshed library (2), assuming the license there is compatible with 3-clause BSD we may be able to use that.

To Do:

• Add strict mypy checking to the CI.
• Determine what might need to be done in the 2 vs. 3 case (I think they may be slightly different)

Modules to cover:

• easter
• parser
• relativedelta
• rrule
• tz
• zoneinfo

I am not sure if we want to do this entirely inline or with stubs - anyone who knows mypy maybe can chime in on the benefits of each. I would think the ideal would be to use the inline comment-style hinting, but I'm not sure how that works with 2/3 polyglot code.

cc @TheGhostHuCodes who has volunteered to help out with this effort.