[3.13] gh-113841: fix possible undefined division by 0 in _Py_c_pow() (GH-127211...

[3.13] gh-113841: fix possible undefined division by 0 in _Py_c_pow() (GH-127211) (#127216)

Note, that transformed expression is not an equivalent for original one (1/exp(-x) != exp(x) in general for floating-point numbers). Though, the difference seems to be ~1ULP for good libm implementations.

It's more interesting why division was used from beginning. Closest algorithm I've found (no error checks, of course;)) - it's Algorithm 190 from ACM: https://dl.acm.org/doi/10.1145/366663.366679. It uses subtraction in the exponent.

(cherry picked from commit f7bb658124aba74be4c13f498bf46cfded710ef9)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>