add linting stubs with flake8-pyi and check for unused imports (#186)

django-stubs/core/files/uploadhandler.pyi

@@ -8,8 +8,8 @@ from django.utils.datastructures import MultiValueDict
 class UploadFileException(Exception): ...
 
 class StopUpload(UploadFileException):
-    connection_reset = ...  # type: bool
-    def __init__(self, connection_reset: bool = False) -> None: ...
+    connection_reset: bool = ...
+    def __init__(self, connection_reset: bool = ...) -> None: ...
 
 class SkipFile(UploadFileException): ...
 class StopFutureHandlers(UploadFileException): ...