Bind method may easily fail on Unix if there is no permission to bind to a privileged port:

>>> try: TCPServer(("", 80), ...)
... except Exception as err: err
... 
PermissionError(13, 'Permission denied')
>>> gc.collect()
__main__:1: ResourceWarning: unclosed <socket.socket fd=3, family=AddressFamily.AF_INET, type=SocketType.SOCK_STREAM, proto=0, laddr=('0.0.0.0', 0)>
0

This problem is inherited by HTTPServer and WSGIServer. My current workaround includes this code in a BaseServer fixup mixin, invoking server_close() if __init__() fails:

class Server(BaseServer, Context):
    def __init__(self, ...):
        try:
            super().__init__((host, port), RequestHandlerClass)
        except:  # Workaround for socketserver.TCPServer leaking socket
            self.close()
            raise
    
    def close(self):
        return self.server_close()