Drop Python 3.9 support

[jameslamb]

Description

Contributes to rapidsai/build-planning#88

Finishes the work of dropping Python 3.9 support.

This project stopped building / testing against Python 3.9 as of rapidsai/shared-workflows#235.
This PR updates configuration and docs to reflect that.

Notes for Reviewers

How I tested this

Checked that there were no remaining uses like this:

git grep -E '3\.9'
git grep '39'
git grep 'py39'

And similar for variations on Python 3.8 (to catch things that were missed the last time this was done).

[jakirkham]

Thanks James! 🙏

Can confirm the only remaining references are in the CHANGELOG.md (as expected)

$ git grep '3\.9'                    
CHANGELOG.md:- Update minimum Python version to Python 3.9 ([#1164](https://github.com/rapidsai/dask-cuda/pull/1164)) [@shwina](https://github.com/shwina)
CHANGELOG.md:- Update minimum Python version to Python 3.9 ([#1164](https://github.com/rapidsai/dask-cuda/pull/1164)) [@shwina](https://github.com/shwina)

[jakirkham]

/merge