dateutil not required for 3.11+

We can replace isoparse conditionally with https://docs.python.org/3/library/datetime.html#datetime.datetime.fromisoformat. Requires a small change to the pyproject.toml as well. Should remove dep on six which makes my brain light up.

Begin with pyproject.toml and search the repository for isoparse and six; read the surrounding date-parsing entry points and check how Python 3.6+ compatibility is handled. Done means the conditional datetime.fromisoformat path works for supported versions, unnecessary dependencies are removed from pyproject.toml, and the existing test suite passes.

Written by the indexing model from the issue text.