Support Python 3.13, drop Python 3.8

[jvstme]

This commit includes:

• Updated version constraints
• Running tests on 3.9-3.13
• Changes by ruff format using new 3.9 syntax

Closes #1833

[jvstme]

There are warnings when running tests under 3.13 that I couldn't find an easy solution for and decided to leave as is for now. The warnings don't seem to appear outside of tests.

  /usr/local/lib/python3.13/site-packages/pydantic/typing.py:68: DeprecationWarning: Failing to pass a value to the 'type_params' parameter of 'typing.ForwardRef._evaluate' is deprecated, as it leads to incorrect behaviour when calling typing.ForwardRef._evaluate on a stringified annotation that references a PEP 695 type parameter. It will be disallowed in Python 3.15.
    return cast(Any, type_)._evaluate(globalns, localns, recursive_guard=set())

This looks like pydantic/pydantic#9613 and may be fixed by #1844

src/tests/_internal/proxy/routers/test_service_proxy.py::test_auth[None-403]
  /usr/local/lib/python3.13/asyncio/base_events.py:746: RuntimeWarning: coroutine method 'aclose' of 'make_app.<locals>.DependencyInjector.get_repo' was never awaited
    self._ready.clear()

This looks similar to python/cpython#117536, yet that bug is indeed fixed.