gh-136065: Fix quadratic complexity in os.path.expandvars() - #134952

Label error. Requires exactly 1 of: type-feature. Found: type-security, awaiting core review, needs backport to 3.9, needs backport to 3.10, needs backport to 3.11, needs backport to 3.12, needs backport to 3.13, needs backport to 3.14