cleanup setup

• remove setup.py test command based on pytest best practices
• use pathlib API
• Update required Python to 3.8 (earlier versions are past EoL), and update minimal requirements according what is provided on conda for Python >= 3.8
• Switch docker containers from conda to micromamba (faster and still maintained conda client).
• Use packaging to parse versions (it is a dependency of setuptools, so it comes for free)
• Use python_requires setup kwarg, to rely on built-in python version checking
• Use arparse for more readable parsing of argv.
• Switch to ctypes.util.find_library to search for mumps. It seems to be more robust than the previous version, which was more dependent on environment variables I believe.
• Reduce residual dependence on distutils