Switch `toml` TOML library for `tomli` (#2301)

toml unfortunately has a lack of maintainership issue right now. It's
evident by the fact toml only supports TOML v0.5.0. TOML v1.0.0 has
been recently released and right now Black crashes hard on its usage.

tomli is a brand new parse only TOML library. It supports TOML
v1.0.0. Although TBH we're switching to this one mostly because
pip is doing the same.

*The upper bound was included at the library maintainer's request.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>
Co-authored-by: Taneli Hukkinen <3275109+hukkin@users.noreply.github.com>