git-p4: use dict.items() iteration for python3 compatibility

Python3 uses dict.items() instead of .iteritems() to provide
iteratoration over dict.  Although items() is technically less efficient
for python2.7 (allocates a new list instead of simply iterating), the
amount of data involved is very small and the penalty negligible.

Signed-off-by: Yang Zhao <yang.zhao@skyboxlabs.com>
Reviewed-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -1745,7 +1745,7 @@ class P4Submit(Command, P4UserMap):
                 break
         if not change_entry:
             die('Failed to decode output of p4 change -o')
-        for key, value in change_entry.iteritems():
+        for key, value in change_entry.items():
             if key.startswith('File'):
                 if 'depot-paths' in settings:
                     if not [p for p in settings['depot-paths']