pycompat.iteritems() just calls .items().

This commit applies a regular expression search and replace to convert
simple instances of pycompat.iteritems() with .items(). There are still
a handful of calls to pycompat.iteritems() remaining. But these all have
more complicated expressions that I wasn't comfortable performing an
automated replace on. In addition, some simple replacements were withheld
because they broke pytype. These will be handled by their own changesets.

Differential Revision: https://phab.mercurial-scm.org/D12318