Add missing pytest-asyncio plugin (#461)

Prevent pytest skipping async tests and issuing
PytestUnhandledCoroutineWarning. Also prevent PytestConfigWarning due to
asyncio_default_fixture_loop_scope config option in pyproject.toml.
Also eliminate "PytestUnknownMarkWarning: Unknown pytest.mark.asyncio"

ci/environment.yml

@@ -22,6 +22,7 @@ dependencies:
   - mypy
   - ruff
   - pandas-stubs
+  - pytest-asyncio
   - pytest-mypy
   - pytest-cov
   - pytest