ContextVar LookupError on some requests

[tim-schilling]

There's a case where the usage of self._ctx.get() is throwing a LookupError.

I'm unable to reliably reproduce the issue.

LookupError: <ContextVar name='__scout_trackedrequest' at 0x7f241cd17ae0>
  File "django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "scout_apm/django/middleware.py", line 85, in __call__
    return self.get_response(request)
  File "contextlib.py", line 124, in __exit__
    next(self.gen)
  File "scout_apm/core/tracked_request.py", line 132, in span
    self.stop_span()
  File "scout_apm/core/tracked_request.py", line 124, in stop_span
    self.finish()
  File "scout_apm/core/tracked_request.py", line 169, in finish
    context.clear_tracked_request(self)
  File "scout_apm/core/context.py", line 136, in clear_tracked_request
    if self._ctx and self._token is not None and self._ctx.get() is instance: