Remove some unused imports
odict.py: remove usage of iteritems_compat which was not working

I've added iteritems_compat function in one of my previous pull
requests, which was failing with:

    ./markdown/odict.py:106: undefined name 'iteritems_compat'

This commit drops usage of that function.
Merge pull request #207 from mitya57/cleanup

Some minor code cleanups