Make import checks properly detect blank lines and comments

Fixes H306 and H305, to do the right thing in the following case:

 import sys

 # this is a comment
 import six

Where hacking should detect there are spaces between import sys and
import six.

Based in pep8 commit: f45d3020c9e38aecc2f736ab274b4bf759319cad

Change-Id: Ie6711bd787b2978d1778c6f0e3d3ea4f571307db