Introduce 3.15 and drop 3.10 Python image generation

Python pre-releases

• Use rc tag, unstable Python version
• Try to build image
• Introduce with build dependencies installed in the meantime
  • gcc required to build msgpack and other dependencies required otherwise assert.h is missing
  • Alpine: build-base or alpine-sdk
• Try to build Silkaj and DuniterPy

Python stable release

• Remove alpine-sdk once no longer necessary, once Py3.15 wheels are provided to run poetry install
  • Once msgpack provides Py3.15 wheels
• Remove rc tag once Python 3.15 is released as stable
• Drop Python 3.10 image generation

Post

• Create follow-up ticket for Py 3.16/3.11