Add __qualname__ property to allow migration without problems

When django generate migration, it does a call do deconstruct, which recreate the models.
ref: https://github.com/django/django/blob/master/django/db/migrations/operations/models.py#L74

When calling the __qualname__ attribute, a <local>.Mixin si returned and this can't be instanciated.
Return the correct way to call in the __qualname__ attribute.

Close bug #42.

Author: egguy

django_prometheus/models.py

@@ -1,4 +1,5 @@
 from prometheus_client import Counter
+from functools import wraps
 
 from django_prometheus.conf import NAMESPACE
 
@@ -37,7 +38,7 @@ class User(ExportModelOperationsMixin('user'), Model):
     model_updates.labels(model_name)
     model_deletes.labels(model_name)
 
-    class Mixin:
+    class Mixin(object):
         def _do_insert(self, *args, **kwargs):
             model_inserts.labels(model_name).inc()
             return super()._do_insert(*args, **kwargs)
@@ -50,4 +51,5 @@ def delete(self, *args, **kwargs):
             model_deletes.labels(model_name).inc()
             return super().delete(*args, **kwargs)
 
+    Mixin.__qualname__ = "ExportModelOperationsMixin('{}')".format(model_name)
     return Mixin