Migrate to pytest-asyncio 0.19

Starting from pytest-asyncio>=0.17, three modes are provided: auto, strict and legacy. Starting from pytest-asyncio>=0.19 the strict mode is the default.

In auto mode, the pytest.mark.asyncio marker can be omitted, the marker is added automatically to async test functions.

See documentation: https://pypi.org/project/pytest-asyncio/0.19.0/