Pytest-asyncio deprecation warning on asyncio_mode

[davesteele]

Python 3.10 is issuing a deprecation warning on pytest asyncio_mode:

../../../../usr/lib/python3/dist-packages/pytest_asyncio/plugin.py:191
  /usr/lib/python3/dist-packages/pytest_asyncio/plugin.py:191: DeprecationWarning: The 'asyncio_mode' default value will change to 
'strict' in future, please explicitly use 'asyncio_mode=strict' or 'asyncio_mode=auto' in pytest configuration file.
    config.issue_config_time_warning(LEGACY_MODE, stacklevel=2)

-- Docs: https://docs.pytest.org/en/stable/warnings.html

The stderr output can lead to autopkgtest failures.

Addressed by this patch.