Bug fix of pygame.PixelArray involving returned repr string

For Python 3.x, the pixelarray.c function Text_ConcatAndDel returned NULL
instead of the concatenated Unicode string. This caused the tp_repr slot
function _pxarray_repr to also return NULL instead of a string representation,
triggering an exception.
Possible fix for issue #175, uncomfirmed

In the pixelarray.c function _pxarray_subscript, an integer object returned
by PyNumber_Index was not released.