gh-123284: fix _pylong decimal context creation

[picnixz]

This also makes _pylong no longer leak references due to the contextvars in decimal, although _pydecimal (and _decimal) will still leak once we call decimal.getcontext().

• Issue: Creation of an internal decimal context for _pylong has side effects on decimal #123284

[vstinner]

This change is maybe hidden the problem, but I don't think that it's a fix: #123284 (comment)

I don't think that _pylong.py code is wrong.

[picnixz]

Converting into draft until I solved the original issue.

[picnixz]

Actually, this will be fixed by #123703 because what's missing the Py_VISIT on the signals and cond maps.