Python 3.5 will change the default value of convert_charrefs, so 3.4
gives warnings if it's not present. This is slightly technical as 2.7
doesn't have the kwarg. Thankfully, we already have a bunch of
workarounds for different versions.
The problem description in #18239 asserted that
http://bugs.python.org/issue670664 was fixed in Python 2.6.8, but based on
http://bugs.python.org/issue670664#msg146770 it appears that's not correct; the
fix was only applied in 2.7, 3.2, and Python trunk. Therefore we must use our
patched HTMLParser subclass in all Python 2.6 versions.