Revise outdated Python 2 comment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:

h11/h11/_receivebuffer.py

Lines 23 to 43 in fb6c715

	# Note that starting in Python 3.4, deleting the initial n bytes from a
	# bytearray is amortized O(n), thanks to some excellent work by Antoine
	# Martin:
	#
	# https://bugs.python.org/issue19087
	#
	# This means that if we only supported 3.4+, we could get rid of the code here
	# involving self._start and self.compress, because it's doing exactly the same
	# thing that bytearray now does internally.
	#
	# BUT unfortunately, we still support 2.7, and reading short segments out of a
	# long buffer MUST be O(bytes read) to avoid DoS issues, so we can't actually
	# delete this code. Yet:
	#
	# https://pythonclock.org/
	#
	# (Two things to double-check first though: make sure PyPy also has the
	# optimization, and benchmark to make sure it's a win, since we do have a
	# slightly clever thing where we delay calling compress() until we've
	# processed a whole event, which could in theory be slightly more efficient
	# than the internal bytearray support.)

Originally posted by @hugovk in #114 (comment)