config: make config.items() return a copy

config.items() was iterating over a copy of the data for the the
specified section on Python 2 by using .items(). However, on Python 3,
items() does not make a copy, so let's switch to explicitly making a
copy to make it safe on both Python 2 and Python 3.

mercurial/config.py

@@ -68,7 +68,7 b' class config(object):'
     def sections(self):
         return sorted(self._data.keys())
     def items(self, section):
-        return self._data.get(section, {}).items()
+        return list(self._data.get(section, {}).iteritems())
     def set(self, section, item, value, source=""):
         if pycompat.ispy3:
             assert not isinstance(value, str), (