chore(test): set default asyncio test fixture scope to function

this is annoying...
a warning is issued without this present.
the change to `pytest-asyncio` was added here:
pytest-dev/pytest-asyncio@c74d1c3#diff-b6eecb9684dfb3a8e0ad8e43510999af4dc5dc5d03edaafd7fe6f872b22fe857R211

python/nwis_client/pytest.ini

@@ -1,4 +1,4 @@
 [pytest]
 markers =
     slow: marks tests as slow (deselect with '-m "not slow"')
-    
+asyncio_default_fixture_loop_scope = function