Some minor code cleanups

This removes some unused imports and fixes a bug introduced by me in #201 (I don't know a way to reproduce it, maybe that code is unused at all).

commit 299bb08bfe203dff0843b9ec9b9c565a63be1c4c:

    Remove some unused imports

commit c10dfac9909282b54a48be6261f76ba11d9da6d0:

    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.