Tidying up in cli/utils/_plotting

[ajjackson]

    from operator import is_none
    from typing import Any
    from toolz.dicttoolz import valfilter

    mapping = {'cmap': 'image.cmap',
               'fontsize': 'font.size',
               'font': 'font.sans-serif',
               'linewidth': 'lines.linewidth',
               'figsize': 'figure.figsize'}

    def is_not_none(item: Any) -> bool:   # Python 3.14 adds this to operator
        return not is_none(item)

    # Explicit args take priority over any other
    args_dict = valfilter(is_not_none, vars(user_args))

    explicit_args = {
        mapping[key]: args_dict[key]
        for key in (args_dict.keys() & mapping.keys())
    }

Let's note that for another PR

Originally posted by @ajjackson in #481 (comment)