Switch from `toml` to `tomli`

[sobolevn]

toml does not seem to be maintained: uiri/toml#361
Instead tomli was created: https://github.com/hukkin/tomli

mypy has already switched to tomli: python/mypy#10824

[sobolevn]

After the next mypy release (after 0.910) we can remove this line: $(grep tomli== requirements-tests-py3.txt) in

typeshed/.github/workflows/tests.yml

Line 80 in 7bfdacb

- run: pip install $(grep tomli== requirements-tests-py3.txt) $(grep mypy== requirements-tests-py3.txt)