Łukasz: what would you recommend for downstream packagers?

I have essentially two options (assuming this isn't reverted in cpython master which I believe makes the most sense since cpython still works fine with older libmpdec):

- revert this individual commit as a patch
- fork further from debian's packaging and use the vendored libmpdec (this potentially carries other negative side-effects since it makes the packages unlike upstream's -- and potentially susceptible to vendor/security drift since we don't carry patches for vendored components (but would get them from system libmpdec))

I'd rather not carry that patch indefinitely if possible -- especially when this cannot be built in a compatible way on software that's a mere 2 months old (ubuntu 20.04).  For similar libraries (ssl, ffi, etc.) this would be a very major break in building.