Add "libmpdec" to build deps on 3.15+

[tianon]

This unlocks the _decimal fast-path for the decimal stdlib module, making Decimal arithmetic ~16x faster (or rather, avoiding a 16x slowdown regression compared to 3.14 and below).

• Fixes 3.15 images ship without _decimal (missing libmpdec-dev) #1136
• See also Unvendor libmpdec sources python/cpython#115119
• If we merge Add _decimal to python-imports test official-images#22503 first, we can re-run CI here and test that it fixes the failures.

[yosifkit]

That's strange. libmpdec-dev (and the whole source package) exists in Trixie (stable) and Bullseye (oldoldstable) but not Bookworm (oldstable) (https://packages.debian.org/trixie/libmpdec-dev).

https://packages.debian.org/search?suite=bookworm&searchon=contents&keywords=libmpdec.so

[yosifkit]

I think we need a few changes, and which we choose for 3.16 could alter the test changes.

• for 3.15: add --without-system-libmpdec to bookworm
  • it is still included, just not used by default
• for 3.16: either drop bookworm or just leave it as slow 🤷

[yosifkit]

Ah, it was dropped in Bookworm because it had nothing depending on it: https://bugs.debian.org/1056785. Apparently, python was the last package that did. ♻️😆

Please remove mpdecimal, cypthon now using the library included in the
cpython source. There are no other users in the archive.