test: configure asyncio_default_fixture_loop_scope

Set it to function, the narrowest scope, which the docs say will be the
default in future. This will silence deprecation warnings about this
variable not being set, and may help with the unit test failures on
python 3.9 currently.

pytest.ini

@@ -0,0 +1,3 @@
+[pytest]
+asyncio_default_fixture_loop_scope = function
+