[1.6.x] Improved strip_tags and clarified documentation

The fact that strip_tags cannot guarantee to really strip all
non-safe HTML content was not clear enough. Also see:
https://www.djangoproject.com/weblog/2014/mar/22/strip-tags-advisory/
Backport of 6ca6c36f8 from master.
[1.6.x] Added release note about strip_tags improvement

Backport of 1c8dbb0cc from master.
[1.6.x] Removed a strip_tags test for older Python versions

Django's custom HTMLParser for older Python versions cannot
parse convoluted syntax.
[1.6.x] Tweaked strip_tags tests to pass on Python 3.3

Backport of 6a0291bda from master.