I'd love it if we could make this happen for Python 3.11. What can I do to help, e.g. would it be helpful if I drafted a PEP?
Several projects have switched to using tomli (https://github.com/hukkin/tomli), which is fully compatible with TOML v1.0.0 and is maybe 800 LoC with claimed 100% branch coverage.
There's also tomlkit, which has been around for longer, but is perhaps larger and more feature-ful than we'd want in stdlib, e.g. it allows you to roundtrip dump and load TOML documents preserving comments, whitespace, etc.