Update EasyAuditMiddleware to support async context

Replaced standard threading with 'asgiref.local' in EasyAuditMiddleware. Also, made EasyAuditMiddleware extend Django's MiddlewareMixin to automatically handle sync and async execution modes.
Github issue: soynatan#291

easyaudit/middleware/easyaudit.py

@@ -1,6 +1,7 @@
 # makes easy-audit thread-safe
+from asgiref.local import Local
 import contextlib
-from threading import local
+from django.utils.deprecation import MiddlewareMixin
 
 
 class MockRequest:
@@ -10,7 +11,7 @@ def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
 
 
-_thread_locals = local()
+_thread_locals = Local()
 
 
 def get_current_request():
@@ -37,12 +38,9 @@ def clear_request():
         del _thread_locals.request
 
 
-class EasyAuditMiddleware:
+class EasyAuditMiddleware(MiddlewareMixin):
     """Makes request available to this app signals."""
 
-    def __init__(self, get_response=None):
-        self.get_response = get_response
-
     def __call__(self, request):
         _thread_locals.request = (
             request  # seems redundant w/process_request, but keeping in for now.