[3.13] gh-127020: Make `PyCode_GetCode` thread-safe for free threading (GH-127043) (GH-127107)

Some fields in PyCodeObject are lazily initialized. Use atomics and
critical sections to make their initializations and accesses thread-safe.
(cherry picked from commit 3926842117feffe5d2c9727e1899bea5ae2adb28)

Co-authored-by: Sam Gross <colesbury@gmail.com>