[gyb] Work-around PEP 3106 for Python 3 compatibility

PEP 3106 [1] changed the behavior of the dictionaries `items` method.
In Python 2, `items` builds a real list of tuples where `iteritems`
returns a generator. PEP 3106 changes Python 3's `items` method to be
equivalent to Python 2's `iteritems` and completely removes `iteritems`
in Python 3.

This patch switches to both to use `items`. This could have a negative
impact on Python 2's performance because it now causes the dictionary
tuples to be built in memory.

[1] https://www.python.org/dev/peps/pep-3106/

utils/GYBUnicodeDataUtils.py

@@ -66,7 +66,7 @@ class GraphemeClusterBreakPropertyTable(UnicodeProperty):
         # values to symbolic values.
         self.symbolic_values = \
             [ None ] * (max(self.numeric_value_table.values()) + 1)
-        for k,v in self.numeric_value_table.iteritems():
+        for k,v in self.numeric_value_table.items():
             self.symbolic_values[v] = k
 
         # Load the data file.