additional mkPython Arguments

We have found a need to add another escape hatch,
to work around mach-nix troubles
by accepting arbitrary nix code for additonal_mkPython_arguments,
for example to get python 3.10 and anything using black running right now you need to add

[python]
additional_mkpython_arguments = """
providers.platformdirs="nixpkgs";
providers.tomli=nixpkgs";
""" # must be verbatim nix code

Also, upgraded the just_python example to 3.10.