[1.4.x] Fix an HTML-parser test that's failed in Python 2.6.8 since 5c79dd58.

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.

Backport of fcec904e from master. Fixes #19148.