Remove backported urllib, html, http modules (for now)

* These were broken (failing lots of tests). More thought needed.
Only run generic import tests on Py2
Improve comments in libfuturize.fixes
Add subset of 3to2 to libfuturize

* Also refactor to create two separate dirs of fixes: fixes2 and fixes3. These
  represent fixes from each Python version to a ``future``-enabled common
  subset.