Fix the type annotation for 'argmap'

[sirosen]

The ArgMap type alias defined the mapping option (for
Schema.from_dict) to allow Field or type[Field], but this is
incorrect. The following construction does not work:

Schema.from_dict({"a": fields.Str})

Correct by simply removing type[Field] from the alias.

A quick included improvement is to make the handling of type narrowing
for argmap a little bit more uniform in style (we had it 3 different
ways in different places), and to use a cast to replace a type ignore
with something a little easier to read.

[sirosen]

I need to look at our CI to figure out / decide how we want to handle marshmallow-dev being the 4.0 branch. But I'm comfortable ignoring that for today (don't have bandwidth to fix it right now).