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

django-stubs/conf/urls/__init__.pyi

@@ -1,5 +1,5 @@
 # Stubs for django.conf.urls (Python 3.5)
-from typing import Any, Callable, Dict, List, Optional, overload, Tuple, Union
+from typing import Any, Callable, Dict, Optional, overload, Sequence, Tuple, Union
 
 from django.http.response import HttpResponse, HttpResponseBase
 
@@ -10,7 +10,7 @@ handler403: Union[str, Callable[..., HttpResponse]] = ...
 handler404: Union[str, Callable[..., HttpResponse]] = ...
 handler500: Union[str, Callable[..., HttpResponse]] = ...
 
-IncludedURLConf = Tuple[List[Union[URLResolver, URLPattern]], Optional[str], Optional[str]]
+IncludedURLConf = Tuple[Sequence[Union[URLResolver, URLPattern]], Optional[str], Optional[str]]
 
 def include(arg: Any, namespace: str = ..., app_name: str = ...) -> IncludedURLConf: ...
 @overload
@@ -21,5 +21,5 @@ def url(
 def url(regex: str, view: IncludedURLConf, kwargs: Dict[str, Any] = ..., name: str = ...) -> URLResolver: ...
 @overload
 def url(
-    regex: str, view: List[Union[URLResolver, str]], kwargs: Dict[str, Any] = ..., name: str = ...
+    regex: str, view: Sequence[Union[URLResolver, str]], kwargs: Dict[str, Any] = ..., name: str = ...
 ) -> URLResolver: ...

django-stubs/urls/conf.pyi

@@ -1,4 +1,4 @@
-from typing import Any, List, Optional, Tuple, overload, Callable, Dict, Union
+from typing import Any, Optional, Sequence, Tuple, overload, Callable, Dict, Union
 
 from .resolvers import URLResolver, URLPattern
 from ..conf.urls import IncludedURLConf
@@ -6,7 +6,7 @@ from ..http.response import HttpResponseBase
 
 def include(
     arg: Any, namespace: Optional[str] = ...
-) -> Tuple[List[Union[URLResolver, URLPattern]], Optional[str], Optional[str]]: ...
+) -> Tuple[Sequence[Union[URLResolver, URLPattern]], Optional[str], Optional[str]]: ...
 
 # path()
 @overload
@@ -17,7 +17,7 @@ def path(
 def path(route: str, view: IncludedURLConf, kwargs: Dict[str, Any] = ..., name: str = ...) -> URLResolver: ...
 @overload
 def path(
-    route: str, view: List[Union[URLResolver, str]], kwargs: Dict[str, Any] = ..., name: str = ...
+    route: str, view: Sequence[Union[URLResolver, str]], kwargs: Dict[str, Any] = ..., name: str = ...
 ) -> URLResolver: ...
 
 # re_path()
@@ -29,5 +29,5 @@ def re_path(
 def re_path(route: str, view: IncludedURLConf, kwargs: Dict[str, Any] = ..., name: str = ...) -> URLResolver: ...
 @overload
 def re_path(
-    route: str, view: List[Union[URLResolver, str]], kwargs: Dict[str, Any] = ..., name: str = ...
+    route: str, view: Sequence[Union[URLResolver, str]], kwargs: Dict[str, Any] = ..., name: str = ...
 ) -> URLResolver: ...

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())