Fix `type: ignore` in generated schema mixins

Opening this issue to look into fixing type issues that have been suppressed by type: ignore.

Addresses this comment by @joelostblom in https://github.com/vega/altair/pull/4114#pullrequestreview-5178353788:

I think this looks good in general, although one caution I have is that if we broaden ignores too freely we might end up missing something that we would like the type checkers to fix. I don't immediately see any case of that here, but I did notice one place where I wonder if there is a bug in the code that should not have been ignored previously (see inline comment).