Correctly stringify tuple subscripts.

Python turns multiple subscripts separated by commas into a tuple of
subscripts. When stringifying this, Pymbolic inserts the set of
parentheses associated with that tuple inside the square brackets for
the subscript. This commit causes Pymbolic to suppress these
parentheses.
Merge pull request #2 from FInAT/stringify_tuple_subscripts

Correctly stringify tuple subscripts.