[3.11] gh-114563: C decimal falls back to pydecimal for unsupported format strings (GH-114879) (GH-115384)

Immediate merits:
* eliminate complex workarounds for 'z' format support
  (NOTE: mpdecimal recently added 'z' support, so this becomes
  efficient in the long term.)
* fix 'z' format memory leak
* fix 'z' format applied to 'F'
* fix missing 'GH-' format support

Suggested and prototyped by Stefan Krah.

Fixes gh-114563, gh-91060

(cherry picked from commit 72340d15cdfdfa4796fdd7c702094c852c2b32d2)
(cherry picked from commit 09c98e4633848ce05df8621f41eb09954b55217a)

Co-authored-by: Stefan Krah <skrah@bytereef.org>