fix: Don't allow unparenthesizing walruses

[cobaltt7]

Description

Resolves #3758.

Checklist - did you ...

• [y] Add an entry in CHANGES.md if necessary?
• [y] Add / update tests if necessary?
• [-] Add new / update outdated documentation?

[cobaltt7]

It turns out that adding that check to maybe_make_parens_invisible_in_atom causes Black to add parenthesis around walruses where it's unnecessary. See the test failure (we didn't notice it previously since that test doesn't run with --preview). I moved the check to visit_dictsetmaker() where we can be sure it won't affect anything else.

[JelleZijlstra]

Thank you!

[wizpig64]

Thanks @RedGuy12 for fixing my bug!