`copy.deepcopy` code is too nested.

[SobhanMP]

Bug report

Bug description:

Hi, I think that the code for copy.deepcopy is too nested and would be cleaner with the walrus operator.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

P.S.

Sorry about the label.

Linked PRs

• gh-135865: Use the walrus operator in deepcopy #135857