provide a new version of Connection that returns/accepts enum types for PAUSED/NEED_DATA our_role their_role etc

[graingert]

the current Connection.next_event Event | type[_Sentinel] return type is slightly painful to work with. It would be useful to have a version that returns enum based sentinels instead

[graingert]

see also #144 and https://discuss.python.org/t/pep-661-sentinel-values/9126/1