TYP use typeddict to define cssdict
xref https://github.com/pandas-dev/pandas/pull/39942#issuecomment-783363832 - here's a solution, the [docs](https://docs.python.org/3/library/typing.html#typing.TypedDict) say that

> At runtime it is a plain dict
Thanks @attack68 - I've merged master and the tests pass, so looks like they were unrelated @simonjayhawkins ok here? Thanks @MarcoGorelli 