Add a definition for Storage.get_alternative_name (#732)

See: f5802a21c4/django/core/files/storage.py (L65)

django-stubs/core/files/storage.pyi

@@ -8,6 +8,7 @@ class Storage:
     def open(self, name: str, mode: str = ...) -> File: ...
     def save(self, name: Optional[str], content: IO[Any], max_length: Optional[int] = ...) -> str: ...
     def get_valid_name(self, name: str) -> str: ...
+    def get_alternative_name(self, file_root: str, file_ext: str) -> str: ...
     def get_available_name(self, name: str, max_length: Optional[int] = ...) -> str: ...
     def generate_filename(self, filename: str) -> str: ...
     def path(self, name: str) -> str: ...