add typing information

[Kriechi]

closes #368

This PR goes hand in hand with python-hyper/hpack#277 and python-hyper/hyperframe#163 and might be refined together.

This PR is still a draft. The major outstanding issues are around handling of Headers, with their (bytes, bytes) or (str, str) or HeaderTuple types. I'm open to suggestions on how to best address these issues. Currently, mypy still reports about 26 typing-related errors and a few tests are failing.

[Kriechi]

@BYK happy to hear your thoughts on the header typing dilemma!

[Kriechi]

@BYK I think I solved the headers type confusion and now all tests pass again. mypy is almost happy, but I think I would like to hear your input on the few remaining findings as they mostly relate to #1286 :

src/h2/utilities.py:130: error: Argument 1 to "startswith" of "str" has incompatible type "bytes"; expected "str | tuple[str, ...]"  [arg-type]
src/h2/utilities.py:138: error: Argument 1 to "startswith" of "str" has incompatible type "bytes"; expected "str | tuple[str, ...]"  [arg-type]
src/h2/utilities.py:259: error: Argument 1 to "search" of "Pattern" has incompatible type "bytes | str"; expected "Buffer"  [arg-type]
src/h2/utilities.py:604: error: Argument 1 to "append" of "list" has incompatible type "bytes | str"; expected "bytes"  [arg-type]
src/h2/utilities.py:624: error: Unsupported operand types for in ("bytes | str" and "bytes | str")  [operator]
src/h2/utilities.py:625: error: Argument 1 to "split" of "bytes" has incompatible type "bytes | str"; expected "Buffer | None"  [arg-type]
src/h2/utilities.py:625: error: Argument 1 to "split" of "str" has incompatible type "bytes | str"; expected "str | None"  [arg-type]

[BYK]

@Kriechi apologies for the very late response, got busy 😅

Regarding your solution for HeaderTuple and its variations, I'm guessing that's the new Headers thing from hpack/struct which I cannot see on the repo. So I need to see that code first to be able to comment.

Regarding the errors you pointed out, I'm quite sure they'll go away when you merge #1286 as the whole point of that patch was to remove these inconsistencies and lock everything to bytes. Makes sense?

[Kriechi]

@BYK the new Headers is defined here https://github.com/python-hyper/hpack/pull/277/files#diff-e10aaf7dbceff5ef795c06588c5b846012815fd4c2fec5b1008255699e9c7b0bR48

Though, I just noticed this syntax is not yet supported on Python 3.9... so I need to find a different solution.

[BYK]

@Kriechi made a suggestion over there. I think that also takes care of the py3.9 issue as it doesn't have nested Unions?