fix(django): Patch __self__ onto middlewares (#773)

Fix #661

Author: untitaker

sentry_sdk/integrations/django/middleware.py

@@ -91,9 +91,14 @@ def sentry_wrapped_method(*args, **kwargs):
 
             try:
                 # fails for __call__ of function on Python 2 (see py2.7-django-1.11)
-                return wraps(old_method)(sentry_wrapped_method)  # type: ignore
+                sentry_wrapped_method = wraps(old_method)(sentry_wrapped_method)
+
+                # Necessary for Django 3.1
+                sentry_wrapped_method.__self__ = old_method.__self__  # type: ignore
             except Exception:
-                return sentry_wrapped_method  # type: ignore
+                pass
+
+            return sentry_wrapped_method  # type: ignore
 
         return old_method