chore(django-spanner): clean up .coveragerc for minimal 100% coverage setup

Author: ohmayr

packages/django-google-spanner/.coveragerc

@@ -8,13 +8,6 @@
 
 [run]
 branch = True
-source =
-    django_spanner
-
-[paths]
-source =
-    django_spanner
-    */site-packages/django_spanner
 
 [report]
 fail_under = 100
@@ -27,5 +20,6 @@ exclude_lines =
     # Ignore abstract methods
     raise NotImplementedError
 omit =
+  */site-packages/*.py
   tests/*
   */tests/*