Urllib3-future

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- Minor improvements on our algorithm that manage multiplexed connection.
A) We ensured that when a remote peer sent a Goaway frame, we keep the connection alive just long enough that you may
retrieve all remaining data/response pending.
B) HTTP/3 max stream limit was not calculated properly (and in real time) thus causing undesirable additional latency in rare cases.
C) Implement ``is_saturated`` for ``ConnectionPool`` to get a hint on whether all allocatable stream are busy.
- Removed unused code from older version of urllib3-future ``HTTPProtocolFactory.has(...)`` and ``ResolverFactory.has(...)``.
- Fixed using "very-specific" scheme for supported web extension like ``ws+wsproto://...`` for ws=plain websocket and wsproto=implementation.
- Reworking the test suite to revamp our coverage target toward 100%.

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- Fixed a rare issue where Happy-Eyeballs algorithm would not respect timeout for a plain HTTP connection where all available endpoints are unreachable.
- Fixed an issue where a HTTP/2 idle connection would be considered "used/saturated" instead of "idle" when remote expressed wish to goaway.
This issue can lead to a ``traffic_police.OverwhelmedTraffic`` in synchronous context and indefinite hang in asynchronous after awhile.
- Increased default keepalive window to 1h by default for HTTP/2, and HTTP/3.

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- Fixed a thread/task safety issue when closing a SSE extension.
- Fixed a rare case when closing a connection would hang forever in Python 3.8, and 3.9 due to a bug in a "wait for close" internal procedure.
- Improved tests runtime and performance.

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- Added built-in support for Server-Side-Event (or SSE) via a WebExtension.
It is as simple as doing ``pm.urlopen("GET", "sse://sse.dev/test")``. ``sse`` is using https under the hood by default.
To force SSE via plain HTTP, replace ``sse://`` by ``psse://``.
The ``extension`` attribute of produced response will be set, and you will be able to consume event promptly.
See the documentation to learn more.
- Fixed unintentional regression using ``CONNECT`` verb manually outside of its standard usage.
- Fixed using a WebExtension with ``urlopen(..., multiplexed=True)`` from a PoolManager instance.

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- Improved timeout reliability and performance in asynchronous mode.
- Fixed PyPy discrete background watcher stop condition in synchronous mode when the ConnectionPool isn't closed properly.
- Fixed lack of timeout using default system resolver in asynchronous mode.
- Fixed a rare issue when connection tear down ran into an exception in Windows proactor loop mode. ssltransport is freed before what we initially expected.
- Improved reliability of DNS-over-QUIC, and DNS-over-TLS.

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- Improved support for async I/O data reader.
- Fixed non-respect of blocksize when uploading a body in an asynchronous context.