[3.12] Restore decimal context after decimal doctests (GH-120149) (GH-120168)

The modified context caused tests failures in several other tests.
(cherry picked from commit 2d7ff6e0e7d4c08ba84079a5c19a4a485626e1de)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>