One possible fix would be to compute the absolute value
of n as an unsigned long. I *think* the following is
portable and avoids any undefined behaviour coming
from signed arithmetic overflow.

unsigned long absn;
if (n < 0)
	absn = 1 + (unsigned long)(-1-n);
else
	absn = (unsigned long)n;

Might this work?

Perhaps it would also be worth changing the tests
in test_int from e.g.

self.assertEqual((-a).numbits(), i+1)

to

self.assertEqual(int(-a).numbits(), i+1)

This would have caught the -LONG_MAX error.