Modify merge is_transition_block for readability

[paulhauner]

This is subjective, but I found the existing code to be a little hard to read. I don't think it's immediately clear to the non-Python-guru how the order of operations should work here.

Consider this example:

>>> not True and False
False
>>> not (True and False)
True

I'm not particularly concerned about this PR getting merged, it just feels less like a trap this way IMO.

[paulhauner]

Closing this since there's more of this not ordering going on in process_execution_payload and it seems like it's going to be required knowledge.

[mkalinin]

Wdyt about anything like that:

def is_transition_block(state: BeaconState, block_body: BeaconBlockBody) -> bool:
    transition_is_not_completed = not is_transition_completed(state)
    block_has_payload = block_body.execution_payload != ExecutionPayload()
    return transition_is_not_completed and block_has_payload

[paulhauner]

Wdyt about anything like that:

It does read very nicely!