improve python version compatibility management

Presently, there are a couple of evident issues on the management of Python versions.

1. the CI docker image is not pinned to a specific version so it uses the latest available on conda-forge. This caused the version bump to fail on conda-forge: https://github.com/conda-forge/pyfastspm-feedstock/pull/10
2. given the status of the Python releases shown here https://devguide.python.org/versions/ it is appropriate to bring the minimum supported Python version to 3.10.