CPython 3.13 support

[vytas7]

Formalize CPython 3.13 support:

• [x] Make the test suite pass on 3.13. May be blocked on dependencies not being readied for 3.13 yet.
• [x] Add a CPython 3.13 CI gate.
• [x] Build CPython 3.13 wheels. :arrow_right: Will be delivered as part of #2259.
• [x] Advertise support using "trove classifiers".
• [x] Check if anything needs an update in CONTRIBUTING.md.
• [x] Update release notes regarding supported platforms.
• [x] Check for any new warnings emitted when running tests, e.g., whether we are relying on any deprecated functionality that will be removed in future Python versions.
• [x] Check if all the binary test dependencies (such as orjson) are ready.

[vytas7]

One interesting warning that is not identified on other CPython versions is:

tests/test_static.py::test_bad_path[asgi-/static/something..]
  /tmp/falcon/.tox/mintest/lib/python3.13/site-packages/_pytest/unraisableexception.py:85: PytestUnraisableExceptionWarning: Exception ignored in: <test_hello.NonClosingBytesIO object at 0x71d48835e390>

  Traceback (most recent call last):
    File "/tmp/falcon/.tox/mintest/lib/python3.13/site-packages/coverage/collector.py", line 250, in lock_data
      def lock_data(self) -> None:

  TypeError: 'bool' object is not callable

    warnings.warn(pytest.PytestUnraisableExceptionWarning(msg))

It seems it is raised non-deterministically in one of the affected tests, but always traces back to test_hello.py. I'll investigate.

Besides that, there a couple of asyncio warnings about the aclose() coroutine never getting called, but it may be that they were just emitted to stderr before (we do some text after the text in some cases), but not propagated as a real warning.