use the standard library tomllib on sufficiently new python

In python 3.11 it is no longer necessary to vendor (?) a toml
implementation. The vendored one is now part of the standard library.
Simply use that instead.

Benefits include:
- micro-optimization when using the isort library API in an application
  that already uses tomllib, to avoid reading yet another one from
  on-disk
- seamless support for automatically dropping tomli once the minimum
  python version is upgraded, via tools such as pyupgrade
- distributors who patch out isort to use the system tomli, do not
  actually have to package tomli for versions of python that have
  tomllib