gh-135865: Use the walrus operator in deepcopy

[SobhanMP]

Hi, I think the walrus operator's raison d'être was to make code like this less nested. Apologies if you do not accept whitespace fixes.

• Issue: copy.deepcopy code is too nested. #135865

[python-cla-bot]

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[abstractedfox]

Precedent suggests it would be good to have an open issue for this even if it's just a refactor

[SobhanMP]

Will do it tonight!

[ZeroIntensity]

We typically don't accept "cosmetic changes" (changes that just change styling or whatever), but I think this is a reasonable refactor. cc @serhiy-storchaka as the copy maintainer.

[ZeroIntensity]

We don't need blurb entries for internal-only changes.

[picnixz]

I'm not very fond of the "sometimes is not None" check and sometimes falsey check but I'm not sure about it. For instance, if someone uses __deepcopy__ = False and we make a falsey check only, it will fallback to the next branch instead of branching to the if so it would be a change of behavior.

I think we should do an is not None check everytime becausye an explicit falsey attribute shouldn't be accepted (and we should raise an appropriate TypeError).