I did not implement setters as I don't think there's a meaningful usage or need to them.
Hopefully this may help others understand better Timecodes running at different rates and the relations to the wall-clock time.
The second commit is to preserve the drop frame flag on elementary operations.
python3
> tc = Timecode(29.97, '00:00:00:00', force_non_drop_frame=True)
> assert tc.drop_frame is False
> tc = tc + 1
> tc
'00:00:00:01' before: '00:00:00;01'
> tc.drop_frame
False before: True
Thanks for cubicibo for contribution.
As usual, version `1.4.0` is also released to [PyPI](https://pypi.org/project/timecode/1.4.0/) and running `pip install timecode` will install it.