bpo-28598: Support __rmod__ for RHS subclasses of str in % string formatting operations (#51)

When you use `'%s' % SubClassOfStr()`, where `SubClassOfStr.__rmod__` exists, the reverse operation is ignored as normally such string formatting operations use the `PyUnicode_Format()` fast path. This patch tests for subclasses of `str` first and picks the slow path in that case.

Patch by Martijn Pieters.