Remove or enable unconditionally EXTRA_FUNCTIONALITY stuff from the _decimal module

[skirpichev]

Bug report

Bug description:

I suspect this define was added to make some staging area for new functionality. Though, it has no pure-Python alternative, not tested by CI or build bots, has regressions (latest: #152870).

We should either remove this or include unconditionally, with appropriate support from _pydecimal.py.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

[sobolevn]

The very minimal thing is to compile our code with EXTRA_FUNCTIONALITY, because right now it is not even compiled / checked :(

[weixlu]

Yes, this is not even tested by CI, and we have seen at least 2 related issues: #152870 and #142440

[skirpichev]

The very minimal thing is to compile our code with EXTRA_FUNCTIONALITY

The CI has no tests even for some core functionality, like ints 15-bit digits. This can be tested on some build bot, but I believe this just delay a decision.