Maybe : 

Main problem is currently ThreadingHTTPServer which extends socketserver.ThreadingMixIn and HTTPServer don't overload his server_close() method. This method server_close is defined into both parent class and should be both call in implementation to avoid shadow definition.

Second and linked to first problem is the class socketserver.TCPServer which is parent class of HTTPServer and extend of BaseServer don't overload method server_close() properly indeed this overload call self.socket.close() but should also call super().shutdown() to avoid infinite thread join into ThreadingHTTPServer server_close() method.

Open to advice