Remove keyword only argument for RequestsMiddleware

This causes django to fail when attempting to load middleware clases
because handlers are passed in as args.

google/cloud/logging_v2/handlers/middleware/request.py

@@ -42,7 +42,7 @@ def _get_django_request():
 class RequestMiddleware(MiddlewareMixin):
     """Saves the request in thread local"""
 
-    def __init__(self, *, get_response=None):
+    def __init__(self, get_response=None):
         self.get_response = get_response
 
     def process_request(self, request):