MAINT: remove support and testing on Python 3.8...
MAINT: remove support and testing on Python 3.8
According to NEP 29 it is now time to remove support for Python 3.8
This commit removes support for Python 3.8 from the codebase, as well as
removing the tests for Python 3.8 from the CI workflow.
It also updates the `pyproject.toml` file to reflect the removal of
Python 3.8 support.