Update pytest-asyncio to 0.21.0

[pyup-bot]

This PR updates pytest-asyncio from 0.20.3 to 0.21.0.

Changelog

0.21.0

* Drop compatibility with pytest 6.1. Pytest-asyncio now depends on pytest 7.0 or newer.
* pytest-asyncio cleans up any stale event loops when setting up and tearing down the
event_loop fixture. This behavior has been deprecated and pytest-asyncio emits a
DeprecationWarning when tearing down the event_loop fixture and current event loop
has not been closed.

Links

• PyPI: https://pypi.org/project/pytest-asyncio
• Changelog: https://pyup.io/changelogs/pytest-asyncio/
• Repo: https://github.com/pytest-dev/pytest-asyncio