Changed JoinRestrictionType to inherit from ExtensibleEnum

[usbpc]

Fixes the crash I encountered while processing all events in the matrix #ping:maunium.net:

Traceback (most recent call last):
File "/home/user/dev/ping_room_reader/.venv/lib/python3.14/site-packages/mautrix/types/util/serializable.py", line 101, in deserialize
return cls(raw)
File "/usr/lib/python3.14/enum.py", line 708, in __call__
return cls.__new__(cls, value)
~~~~~~~~~~~^^^^^^^^^^^^
File "/usr/lib/python3.14/enum.py", line 1193, in __new__
raise ve_exc
ValueError: 'fi.mau.spam_checker' is not a valid JoinRestrictionType