do not create sender at close

[methane]

No description provided.

[jgoclawski]

This makes perfect sense, not only fixes problem reported in #207, but also removes redundant code.
I suppose it could be even simpler?

def close(self):
    self.acquire()
    try:
        try:
            if self._sender is not None:
                self._sender.close()
                self._sender = None
        finally:
            super(FluentHandler, self).close()
    finally:
        self.release()

Anyway, LGTM 👍

[methane]

            super(FluentHandler, self).close()

FWIW, it is Python 2 code. We can write super().close() as we have dropped Python 2 support.