fix: Reply NotImplemented for OCPP 2.1 actions without a handler

[IncognitoQuack]

Changes included in this PR

Bug fix, follow-up to #744.

Current behavior

#744 added "2.1" to the versions handled by _raise_key_error(), but the action is still looked up in the OCPP 2.0.1 Action enum. OCPP 2.1 has 26 actions that 2.0.1 doesn't have, like NotifyDERAlarm, SetDERControl, BatterySwap and GetTariffs. When a 2.1 ChargePoint receives one of these and no handler is registered, it answers:

[4,"1","NotSupported","Requested Action is not known by receiver",{"cause":"NotifyDERAlarm not supported by OCPP2.1."}]

So the peer is told that a valid OCPP 2.1 action isn't part of OCPP 2.1.

New behavior

The action is looked up in the Action enum of the OCPP version in use. For 2.1 that's ocpp.v21.enums.Action, a superset of the 2.0.1 enum. A valid 2.1 action without a handler now gets the same answer as a 2.0.1 or 1.6 action without a handler:

[4,"1","NotImplemented","Request Action is recognized but not supported by the receiver",{"cause":"No handler for NotifyDERAlarm registered."}]

Actions that aren't part of OCPP 2.1 still get NotSupported. Nothing changes for 1.6 and 2.0.1. I also replaced the two copies of the try/except block with a single lookup table.

Impact

No breaking changes. The only behavior change is the error code for the 26 OCPP 2.1-only actions, when no handler is registered.

Checklist

1. Does your submission pass the existing tests?
2. Are there new tests that cover these additions/changes? tests/v21/test_v21_charge_point.py. The BatterySwap, NotifyDERAlarm and SetDERControl cases fail without this change.
3. Have you linted your code locally before submission?