Drop Python 3.9

[crusaderky]

The latest version of several of the libraries that integrate with array-api-compat doesn't support Python 3.9 anymore. This produces the headache of having a de-facto, undocumented minimum supported version of those libraries, which is going to become more and more obsolete as time goes by, unwittingly enforced by the Python 3.9 CI jobs.

First of those issues is #230, which in turn blocks #228.

I think array-api-compat should follow the rest of the industry and formally drop python 3.9 support.

[ev-br]

scikit-learn still supports python 3.9, IIUC