contrib: fix dirstatenonnormalcheck to work in Python 3
This is a redo of D1963 that has the added benefit of not breaking
Python 2. Oops.
# skip-blame because this is bytes prefixes and a s/iteritems/items/
Differential Revision: https://phab.mercurial-scm.org/D1970
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.