Ignore abstract type issue for StorageBackend

Mypy can't understand that, even though this returns an instance of an
abstract base class, the instances will always be subclasses of the
base class and therefore they will have their methods defined.

python/mypy#4717

squash_bot/storage/base.py

@@ -80,6 +80,6 @@ def read_file(file_path: str, file_name: str, create_if_missing: bool = False) -
 
 
 def get_storage_backend() -> StorageBackend:
-    return settings_base.get_class_from_string(
+    return settings_base.get_class_from_string(  # type: ignore[abstract]
         settings_base.settings.STORAGE_BACKEND, StorageBackend
     )()