Python3 import fallbacks for urllib/urllib2
Minor comment changes to Python 2/3 imports

The point of this is to help filter out false positive
changes from the 2to3 imports fixer with grep, e.g.

$ 2to3 --no-diffs -n -w -f imports Bio BioSQL Tests Doc Scripts
$ 2to3 --no-diffs -n -w -f imports -d Bio BioSQL Tests Doc Scripts
$ git diff | grep ^[+-] | grep -v "Python 2" | grep -v "Python 3"
No longer need the 2to3 imports fixer