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

@@ -1,67 +1,67 b''
 #!/usr/bin/env python
 """Test the running system for features availability. Exit with zero
 if all features are there, non-zero otherwise. If a feature name is
 prefixed with "no-", the absence of feature is tested.
 """
 
 from __future__ import print_function
 
 import optparse
 import os, sys
 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()
         except Exception as e:
             print("feature %s failed:  %s" % (name, e))
             failed += 1
     return failed
 
 parser = optparse.OptionParser("%prog [options] [features]")
 parser.add_option("--test-features", action="store_true",
                   help="test available features")
 parser.add_option("--list-features", action="store_true",
                   help="list available features")
 
 def _loadaddon():
     if 'TESTDIR' in os.environ:
         # loading from '.' isn't needed, because `hghave` should be
         # running at TESTTMP in this case
         path = os.environ['TESTDIR']
     else:
         path = '.'
 
     if not os.path.exists(os.path.join(path, 'hghaveaddon.py')):
         return
 
     sys.path.insert(0, path)
     try:
         import hghaveaddon
     except BaseException as inst:
         sys.stderr.write('failed to import hghaveaddon.py from %r: %s\n'
                          % (path, inst))
         sys.exit(2)
     sys.path.pop(0)
 
 if __name__ == '__main__':
     options, args = parser.parse_args()
     _loadaddon()
     if options.list_features:
         list_features()
         sys.exit(0)
 
     if options.test_features:
         sys.exit(test_features())
 
     hghave.require(args)