Pass correct registry to MultiProcessCollector (#1152)

`registry` does not exists in prometheus_client.registry, as that causes
an ImportError the test was skipped in the 3.9 scenario.

Signed-off-by: Jelle van der Waa <jelle@vdwaa.nl>

Author: jelly

prometheus_client/django/exposition.py

@@ -6,7 +6,6 @@
 import prometheus_client
 from prometheus_client import multiprocess
 from prometheus_client.exposition import _bake_output
-from prometheus_client.registry import registry
 
 
 class PrometheusDjangoView(View):
@@ -17,7 +16,7 @@ def get(self, request, *args, **kwargs):
         if self.registry is None:
             if self.multiprocess_mode:
                 self.registry = prometheus_client.CollectorRegistry()
-                multiprocess.MultiProcessCollector(registry)
+                multiprocess.MultiProcessCollector(self.registry)
             else:
                 self.registry = prometheus_client.REGISTRY
         accept_header = request.headers.get("Accept")