add static type annotations

[davidism]

Add static type annotations and export them by adding the py.typed marker file. Passes mypy (strict), pyright (basic, strict is impossible), and pyright --verifytypes (checks that exported types are complete). Enables the tox "typing" env by default that runs all three checks, as well as testing in CI.

[davidism]

This is the last of the big PRs. Along with #257 and #258, this now matches how other Pallets projects are organized.

[macnewbold]

This looks great to me. Initially I was concerned for python 3.8 compatibility, but so far as I can tell, I think we're covered.

[davidism]

from __future__ import annotations causes all annotations to be treated as strings initially by Python, so newer syntax and features will only be interpreted by static tools.

[jeffwidman]

Do you plan to cut a new release now that these have landed to get the type annotations live?

Again, many thanks for all the great improvements here. I learned quite a bit just by reading through the PR's.

[davidism]

I'll make a PR to demonstrate how the release workflow works, I think 0.16 is appropriate even though behavior didn't really change from 0.15. Maybe after that start thinking about what 1.0 might look like. 1.0 doesn't have to be perfect or zero inbox or anything like that, but this extension seems like it's stable and maintained at this point.

[jeffwidman]

Sounds good.

We haven't done semver historically because it was just one more thing to have to worry about, but that friction is minimal now since it's a very stable/low-churn extension. We really haven't touched it much the last few years other than updating deprecations/dependencies.

Maybe even make do a 1.0 release instead of 0.16... this is as good a time as any... "now with type annotations!" 😄