KeyError in can_exit_to caused by lru_cache

As pastly reported in https://github.com/pastly/simple-bw-scanner/issues/198, stem giving this traceback:

File "/home/pastly/src/simple-bw-scanner/venv-editable/lib/python3.5/site-packages/stem/exit_policy.py", line 285, in can_exit_to

if not self.is_exiting_allowed():KeyError: (<stem.exit_policy.MicroExitPolicy object at 0x7f8de6438ba8>,)

Apparently caused by  a Python bug with lru_cache.

pastly is already trying to find out which Python versions fail, to use stem's lru_cache for those ones.

This ticket blocks #26848 (moved)

Edit: blocks ticket