Fix mysql_defaults_file fixture

Author: nicoddemus

pytest_services/mysql.py

@@ -16,7 +16,7 @@ def mysql_defaults_file(
         run_services, tmp_path_factory, memory_temp_dir, request):
     """MySQL defaults file."""
     if run_services:
-        cfg = tmp_path_factory.mktemp(request.session.name)
+        cfg = tmp_path_factory.mktemp("pytest-services")
         defaults_path = str(cfg / 'defaults.cnf')
 
         with open(defaults_path, 'w+') as fd:
@@ -27,6 +27,7 @@ def mysql_defaults_file(
 default-time-zone = SYSTEM
     """.format(user=os.environ['USER'], tmpdir=memory_temp_dir))
         return defaults_path
+    return None
 
 
 @pytest.fixture(scope='session')