In CPython, str.add directly throws a TypeError since it is implemented
as a sq_concat.  sqlalchemy relies on being able to do str()+MyStr() and
use MyStr.__radd__ to handle the addition.  We were throwing a TypeError
but not implementing it in sq_concat.

So instead, return NotImplemented instead of directly throwing a TypeError.
This makes '+' more compatible but str.__add__ a bit less compatible.