twisted.python.log.PythonLoggingObserver is broken on Ubuntu 20.04

[Colin Watson]

On up-to-date focal with python3-twisted 18.9.0-11ubuntu0.20.04.1, following https://twistedmatrix.com/documents/current/core/howto/logging.html#using-the-standard-library-logging-module:

  $ python3
  Python 3.8.10 (default, Nov 26 2021, 20:14:08)
  [GCC 9.3.0] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import logging
  >>> from twisted.python import log
  >>> logging.basicConfig(level=logging.INFO)
  >>> observer = log.PythonLoggingObserver()
  >>> observer.start()
  >>> log.msg('test')
  Temporarily disabling observer LegacyLogObserverWrapper(<bound method PythonLoggingObserver.emit of <twisted.python.log.PythonLoggingObserver object at 0x7f962dbc1130>>) due to exception: [Failure instance: Traceback: <class 'TypeError'>: _findCaller() takes from 1 to 2 positional arguments but 3 were given
  <stdin>:1:<module>
  /usr/lib/python3/dist-packages/twisted/python/threadable.py:53:sync
  /usr/lib/python3/dist-packages/twisted/python/log.py:286:msg
  /usr/lib/python3/dist-packages/twisted/logger/_legacy.py:154:publishToNewObserver
  --- <exception caught here> ---
  /usr/lib/python3/dist-packages/twisted/logger/_observer.py:131:__call__
  /usr/lib/python3/dist-packages/twisted/logger/_legacy.py:93:__call__
  /usr/lib/python3/dist-packages/twisted/python/log.py:595:emit
  /usr/lib/python3/dist-packages/twisted/logger/_legacy.py:154:publishToNewObserver
  /usr/lib/python3/dist-packages/twisted/logger/_stdlib.py:115:__call__
  /usr/lib/python3.8/logging/__init__.py:1512:log
  /usr/lib/python3.8/logging/__init__.py:1577:_log
  ]
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>

    File "/usr/lib/python3/dist-packages/twisted/python/threadable.py", line 53, in sync
      return function(self, *args, **kwargs)
    File "/usr/lib/python3/dist-packages/twisted/python/log.py", line 286, in msg
      _publishNew(self._publishPublisher, actualEventDict, textFromEventDict)
    File "/usr/lib/python3/dist-packages/twisted/logger/_legacy.py", line 154, in publishToNewObserver
      observer(eventDict)
  --- <exception caught here> ---
    File "/usr/lib/python3/dist-packages/twisted/logger/_observer.py", line 131, in __call__
      observer(event)
    File "/usr/lib/python3/dist-packages/twisted/logger/_legacy.py", line 93, in __call__
      self.legacyObserver(event)
    File "/usr/lib/python3/dist-packages/twisted/python/log.py", line 595, in emit
      _publishNew(self._newObserver, eventDict, textFromEventDict)
    File "/usr/lib/python3/dist-packages/twisted/logger/_legacy.py", line 154, in publishToNewObserver
      observer(eventDict)
    File "/usr/lib/python3/dist-packages/twisted/logger/_stdlib.py", line 115, in __call__
      self.logger.log(
    File "/usr/lib/python3.8/logging/__init__.py", line 1512, in log
      self._log(level, msg, args, **kwargs)
    File "/usr/lib/python3.8/logging/__init__.py", line 1577, in _log
      fn, lno, func, sinfo = self.findCaller(stack_info, stacklevel)
  builtins.TypeError: _findCaller() takes from 1 to 2 positional arguments but 3 were given

This is https://twistedmatrix.com/trac/ticket/9668; the fix should be a trivial backport.

This caused test failures in launchpad-buildd on focal (though I may look for a workaround).