Compare Py2 vs Py3 better using comparepdf and diffpdf

Procedure:

  virtualenv -p python2.7 .
  rm bin/python
  virtualenv -p python3.4 .
  rm bin/python
  bin/python2 --version  # make sure it prints 2.7
  bin/python3 --version  # make sure it prints 3.4
  bin/python2 -m mgp2pdf -o /tmp/py2 samples/*/*.mgp --unsafe
  bin/python3 -m mgp2pdf -o /tmp/py2 samples/*/*.mgp --unsafe
  cd /tmp
  for i in py2/*.pdf; do j=${i/2/3}; comparepdf -c a $i $j || echo $i $j; done
  diffpdf py2/python3.pdf py3/python3.pdf  # for visual eyeballing