py3: define and use pycompat.iteritems() for hgext/ .iteritems() -> .items() is the last source transform being performed. But it is also the most widely used. This commit adds a pycompat.iteritems symbol and imports it in place of .iteritems() for usage in hgext/. I chose to stop at just hgext/ because the patch will be large and it is an easy boundary to stop at since we can disable source transformation on a per-package basis. There are places where the type does implement items() and we could call items() directly. However, this would require critical thought and I thought it would be easier to just blindly change the code. We know which call sites need to be audited in the future because they have "pycompat.iteritems." With this change, we no longer perform source transformation on hgext! Differential Revision: https://phab.mercurial-scm.org/D7014