Add Trove classifier for Python 3.15

[hugovk]

Request to add a new Trove classifier.

The name of the classifier(s) you would like to add:

• Programming Language :: Python :: 3.15

Why do you want to add this classifier?

Python 3.14 is in beta which means the main branch is now 3.15:

• https://devguide.python.org/versions/

[miketheman]

Thanks!