misc: Drop diff_match_patch

Since it is not used anywhere within TTK itself, the requirement can be
dropped entirely
req: eliminate min-required.txt
req: sort requirements lists
py3: Add six to dependencies
py3: Fix html-related imports on Python 3
cleanup: Replace dictutils.ordereddict by collections.OrderedDict
accesskeys: add properties to bad accesskey detector

Extended the script to check for bad accesskeys in .properties files