tests: move pytest-asyncio dependency in tests/requirements-base.txt

[azmeuk]

This prevents this warnings on some tox test environments:

.tox/py311-django/lib/python3.11/site-packages/_pytest/config/__init__.py:1373
  /home/eloi/dev/authlib/.tox/py311-django/lib/python3.11/site-packages/_pytest/config/__init__.py:1373: PytestConfigWarning: Unknown config option: asyncio_mode

    self._warn_or_fail_if_strict(f"Unknown config option: {key}\n")

These warnings appears because this configuration is shared across all the tox environments:

authlib/tox.ini

Lines 27 to 28 in eaa51a8

	[pytest]
	asyncio_mode = auto