fix: No more BytesWarnings

[BYK]

Fixes #1236.

This patch makes all header operations operate on bytes and converts all headers and values to bytes before operation. With a follow up patch to hpack it should also increase efficiency as currently, hpack casts everything to a str first before converting back to bytes: https://github.com/python-hyper/hpack/blob/02afcab28ca56eb5259904fd414baa89e9f50266/src/hpack/hpack.py#L150-L151

[BYK]

Note to reviewer: I'm aware there are formatting changes (they came from "format on save") and although I think things are more readable now, I'm happy to find and revert all unrelated formatting changes to make reviews easier.

Unfortunately, I realized the auto format changes way too late and wanted to get the patch up as promised first.

Alternatively, I can submit a PR that reformats the entire codebase without any changes and then rebase this on top of that.

Fixed format-related things mostly. I think we should still do a full repo black to make it easier for future contributors.

[Kriechi]

Please keep the changes in each PR to a minimum - in this case please revert the style-related changes.
Discussing coding style is outside of the scope here.

[BYK]

@Kriechi we should be good now, sorry for the trouble.

[Kriechi]

I'm contemplating making the encoding of headers part of the normalization pipeline, instead of being the responsibility of the outside caller.

What do you think about integrating it here?

[BYK]

That's exactly what I wanted but unfortunately we have an option to bypass normalization, which breaks that neat abstraction :(

[Kriechi]

utf8_encode_headers is currently called in two places: connections.py - push_stream and stream.py - send_headers. Both share a code path with H2Stream._build_headers_frames.

I'll try to refactor it and share the update to discuss.

[BYK]

Sounds great!