bpo-41324 Add a minimal decimal capsule API

[skrah]

https://bugs.python.org/issue41324

[pitrou]

Also cc @xhochy just in case he's interested in this.

[pitrou]

Thank you @skrah for this. AFAICT, the API here would be enough for explicitly-typed conversion.

For type inference (I'm not sure that would make sense, but perhaps) there would ideally be an additional API to get the number of digits in a Decimal (that is, len(dec.as_tuple().digits)).

[skrah]

For type inference (I'm not sure that would make sense, but perhaps) there would ideally be an additional API to get the number of digits in a Decimal (that is, len(dec.as_tuple().digits)).

That's a good point. Initially I thought people would use PyDec_Get(v)->digits, but then you need to include
mpdecimal.h. So actually we have two APIs: One for the people who use libmpdec directly and the easier
one. It would make sense to add PyDec_GetDigits(v).

[bedevere-bot]

@skrah: Please replace # with GH- in the commit message next time. Thanks!