Fixes #804: Update PyO3 and test dependencies

[tobiasge]

Fixes #804: This updates PyO3 to 0.28 and the test dependencies to their current major versions.
With these update these warnings from the tests are fixed:

tests/test_asgi.py::test_scope[mt]
.venv/lib/python3.14/site-packages/pytest_asyncio/plugin.py:978
  /home/tgenannt/LocalGit/granian/.venv/lib/python3.14/site-packages/pytest_asyncio/plugin.py:978: DeprecationWarning: 'asyncio.get_event_loop_policy' is deprecated and slated for removal in Python 3.16
    policy = asyncio.get_event_loop_policy()

[tobiasge]

This failing test is strange. All the tests actually passed, but then it waited for something:
https://github.com/emmett-framework/granian/actions/runs/22386217778/job/64797333524?pr=820

[gi0baro]

@tobiasge ignore it, GH actions suck nowadays U.U