Update classifier to support Python 3.13

[shenxianpeng]

Based on CI, pip should already support Python 3.13, so the classifier should also have Python 3.13 included.

pip/.github/workflows/ci.yml

Line 117 in a7369ba

- "3.13"

[webknjaz]

Based on CI, pip should already support Python 3.13, so the classifier should also have Python 3.13 included.

Technically, until Python 3.13 hasn't hit the final release stage itself, it could still break things. But of course, it's up to the pip maintainers who might follow different principles than I do 🤷‍♂️