Use Sequence instead of List for path param (#659)

Unlike `List`, which is invariant, `Sequence` is covariant, which lets
`path` accept lists of subsets of the `Union` as well.

I believe this is safe, as django doesn't mutate this input. I found
[this
comment](https://github.com/python/mypy/issues/3351#issuecomment-300447832)
helpful

tests/typecheck/urls/test_conf.yml

@@ -6,3 +6,13 @@
       def include() -> Tuple[List[Union[URLPattern, URLResolver]], None, None]: ...
 
       path('test/', include())
+
+
+-   case: test_path_accepts_pattern_resolver_union_subset
+    main: |
+      from typing import List, Tuple
+      from django.urls import path, URLPattern
+
+      def include() -> Tuple[List[URLPattern], None, None]: ...
+
+      path('test/', include())