Add URLPattern options to django.url.conf since they were missing. (#583)

* Add URLPattern options to django.url.conf since they were missing.

* Fix some wording in the contribution guide & fix a reference to Django version 3.0, which has since been replaced by 3.1 as new stable version.

* My bad - I was misreading the type hints a bit here. Fix tests.

* We need to specify the List[Union[URLResolver, URLPattern]] upfront, since those don't accept List[URLResolver] as argument.

* Add test to ensure that path() accepts a mix of URLPatterns & URLResolvers.

Co-authored-by: LanDinh <coding+sourcetree@khaleesi.ninja>

tests/typecheck/urls/test_conf.yml

@@ -0,0 +1,8 @@
+-   case: test_path_accepts_mix_of_pattern_and_resolver_output
+    main: |
+      from typing import List, Tuple, Union
+      from django.urls import path, URLPattern, URLResolver
+
+      def include() -> Tuple[List[Union[URLPattern, URLResolver]], None, None]: ...
+
+      path('test/', include())