gh-91291: Accept attributes as keyword arguments in decimal.localcontext

[dignissimus]

This allows attributes to be set like this

with decimal.localcontext(prec=10):
    ...

instead of like this

with decimal.localcontext() as ctx:
    ctx.prec = 10
    ...

#91291

https://bugs.python.org/issue47135

[dignissimus]

Thank you for the review, I'll be able to make the changes later today

[JelleZijlstra]

Looks good apart from one nit which I'll apply myself. Then I'll also run this through the buildbots in case there's a refleak.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @JelleZijlstra for commit df3ab57 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[vstinner]

It seems like this change caused a regression: #91821

[dignissimus]

Oh, interesting. I'll try and work out where the issue is.