Add a test showing that slow, overlapping requests back each other up in HTTP/2.

[nmlorg]

Summary

Quick proof of concept related to encode/httpx#3278 (and possibly #778).

As committed:

$ pytest tests/*/test_http2.py
⋮
tests/_async/test_http2.py ....................F
tests/_sync/test_http2.py ..........F
⋮
>       assert fetches == [1.0] * 5
E       assert [1.4, 1.4, 1.2, 1.8, 1.0] == [1.0, 1.0, 1.0, 1.0, 1.0]

With my work in progress:

$ pytest tests/_sync/test_http2.py
⋮
tests/_sync/test_http2.py ...........

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.