Fix #83: Stop testing the DOM treewalker with pxdom
Fix #83: Stop testing the DOM treewalker with pxdom

This hasn't actually been tested in years and almost certainly won't
work; furthermore pxdom isn't supported under Python 3 so it doesn't
provide any testing whatsoever for a strict DOM implementation there.

alex/html5lib-python,ordbogen/html5lib-python,mgilson/html5lib-python,mgilson/html5lib-python,mgilson/html5lib-python,ordbogen/html5lib-python,dstufft/html5lib-python,alex/html5lib-python,mindw/html5lib-python,dstufft/html5lib-python,mindw/html5lib-python,alex/html5lib-python,html5lib/html5lib-python,mindw/html5lib-python,html5lib/html5lib-python,ordbogen/html5lib-python,gsnedders/html5lib-python,html5lib/html5lib-python,dstufft/html5lib-python,gsnedders/html5lib-python