Fix some pyflakes-undefined-name warnings

Currently pyflakes reports this:

```
e: python-markdown: pyflakes-undefined-name usr/share/pyshared/markdown/extensions/def_list.py:98: markdown
e: python-markdown: pyflakes-undefined-name usr/share/pyshared/markdown/odict.py:33: copy
e: python-markdown: pyflakes-undefined-name usr/share/pyshared/markdown/odict.py:102: six
e: python-markdown: pyflakes-undefined-name usr/share/pyshared/markdown/odict.py:134: six
```

This PR fixes that. Instead of importing `six`, I copied implementation of `iteritems()` and used that instead.