test_pairwise_swap test case is incorrect

[Chin Pang Leung]

self.assertEqual(bits.pairwise_swap(1), 1) is incorrect. According to the description, the result should be 2.

i.e.
0000000000000001 should result in 0000000000000010 which is 2 instead of 1.

[Emmanuel Arias]

Please, provide an PR to fix it.