BUG: special: comb returned incorrect result when given large np.int64 arguments.

When given `exact=True`, cast the arguments of `comb` to python ints to ensure
that the result is a python integer.  Otherwise a call such as
    comb(np.int64(70), np.int64(30), exact=True)
returns an incorrect result, because it does the calculations with np.int64s.
The true result is too large for a 64 bit integer.