only use __bytes__ when not argument is needed.

[barbibulle]

The use of to_bytes should be reserved for cases where an argument is needed (like int.to_bytes()). For all other cases, __bytes__ should be used.
For legacy reasons, the code base still had a few places where to_bytes was called (and classes where both __bytes__ and to_bytes were defined).
This PR cleans up these cases.

[zxzxwu]

I personally prefer to_bytes since sometimes bytes() brings an additional indent. But it's also ambiguous with int.to_bytes().