[gyb] Convert map object to list object

Python 2's map function [1] returns a list by default. Compared with
Python 3's map function [2] which returns an iterator (or map object).
The former is subscriptable, while the latter is not.

This patch explicitly converts the result of some map operations to be
a list. That way they have the same intended behaviour on both Python 2
and 3.

[1] https://docs.python.org/2/library/functions.html#map
[2] https://docs.python.org/3/library/functions.html#map

utils/GYBUnicodeDataUtils.py

@@ -331,7 +331,10 @@ class UnicodeTrieGenerator(object):
                 else:
                     return idx
 
-            return map(map_index, indexes)
+            # NOTE: Python 2's `map` function returns a list. Where Python 3's
+            # `map` function returns an iterator. To work around this the
+            # result of the `map` is explicitly converted to a `list`.
+            return list(map(map_index, indexes))
 
         # If self.BMP_data contains identical data blocks, keep the first one,
         # remove duplicates and change the indexes in self.BMP_lookup to point to