Why I expected a major speedup from this:  the binary exponentiation routine (for "reasonably small" exponents) does 30 * ceiling(exponent.bit_length() / 30) multiply-and-reduces, plus another for each bit set in the exponent.  That's a major difference between exponents of bit lengths 61 ((P-2).bit_length()) and 1 ((1).bit_length()).  Indeed, I bet it would pay in `long_pow()` to add another test, under the `if (Py_SIZE(b) < 0)` branch, to skip the exponentiation part entirely when b is -1.  `long_invmod()` would be the end of it then.  Because I expect using an exponent of -1 for modular inverse will be overwhelmingly more common than using any other negative exponent with a modulus.