dict is invariant and so we can basically never match the value types in this case. E.g., in

def foo(d: dict[str, int | str]): ...

foo({'a': 3})      # argument is dict[str, int]
foo({'a': 'abc'})  # argument is dict[str, str]

both calls fail to type check because the argument does not exactly match the annotation of foo.
This PR switches to using Mapping which is covariant and so both calls above pass.

There are likely more cases of this in Scipp, but this is at least a start.