py: Protect mp_parse and mp_compile with nlr push/pop block.
To enable parsing constants more efficiently, mp_parse should be allowed to raise an exception, and mp_compile can already raise a MemoryError. So these functions need to be protected by an nlr push/pop block. This patch adds that feature in all places. This allows to simplify how mp_parse and mp_compile are called: they now raise an exception if they have an error and so explicit checking is not needed anymore.