Change comments (mainly URLs) to no longer specifically say Python 3.3

Non-functional changes only. Most of these are URLs that point to docs.python.org/3.3/ and are now going to just /3/ which means "whatever's latest stable on the 3.x branch". There are some remaining references to CPython 3.3 or Python 3.3, eg "inspired by urlparse", and the test suite stipulates a minimum of 3.3; these I haven't changed, for obvious reasons.