There were three problems:
1. MPV.unobserve_property called _mpv_observe_property instead of _mpv_unobserve_property.
2. _mpv_unobserve_property returns the number of properties that were assigned to the handler that is being removed. Because the return value is not 0 in such a case, ErrorCode.raise_for_ec tries to raise an error. To fix that, I changed the ErrorCode.raise_for_ec functon not to raise an error if ec is larger than 0. (If there is a positive return value, there should not have been an error anyway, if I'm not mistaken.)
3. Calling MPV.unobserve_property for a handle that is not currently been used, should not result in a KeyError being raised.
An alternative to 2. would be to add a restype to the _handle_func of 'mpv_unobserve_property' and returning that value in MPV.unobserve_property. (That way, raise_for_ec is never called.) But I don't think this value is useful in any way. Even the built in lua interface does not return that value.
