Buildpack support for Python 3.15 #358

Python 3.15 was released today:
https://blog.python.org/2026/10/python-3150-final-is-here/

We will need to add support to our two Python buildpacks:

• The classic Python buildpack (for Cedar-generation apps): Add support for Python 3.15 heroku-buildpack-python#2159
• The new Python Cloud Native Buildpack (for Fir-generation apps, and also Cedar-generation apps that have switched to the cnb stack): Add support for Python 3.15 buildpacks-python#632