hghave: switch from iteritems to items

With this, test-hghave.t passes on python 3.

Four features fail because mercurial still is not py3 safe:
absimport
cacheable
hardlink
defaultcacerts

But that will be resolved automatically eventually.

tests/hghave

@@ -13,13 +13,13 b' import hghave'
 checks = hghave.checks
 
 def list_features():
-    for name, feature in sorted(checks.iteritems()):
+    for name, feature in sorted(checks.items()):
         desc = feature[1]
         print(name + ':', desc)
 
 def test_features():
     failed = 0
-    for name, feature in checks.iteritems():
+    for name, feature in checks.items():
         check, _ = feature
         try:
             check()