Update version to 0.28.2

[lovelydinosaur]

Prompted by #3524

Update Python package with up to date classifiers.

[codebydivine]

Hello,

I just wanted to point out that httpx no longer work with the latest alpha version of python 3.14: https://www.python.org/downloads/release/python-3140a6/ (alpha 6).

Regards,

[luiz-telles]

Hello, Do you know when this version will be published on PyPI (https://pypi.org/project/httpx/)?

[cclauss]

This branch has conflicts that must be resolved.