Revise outdated Python 2 comment

[hugovk]

Python 2 support was dropped in v0.12.0 (2021-01-01).

Looks like much of this comment could be removed now? There is no longer self._start or self.compress in the file:

https://github.com/python-hyper/h11/blob/fb6c715184c6a1f29e299d2f63d722b2ea4309d5/h11/_receivebuffer.py#L23-L43

Originally posted by @hugovk in https://github.com/python-hyper/h11/issues/114#issuecomment-1114878890