remove unused typevars, make dtype type alias

[MarcoGorelli]

Summary:

• make DType a type alias
• remove several unused TypeVars (we'd agreed to disagree on removing them in https://github.com/data-apis/dataframe-api/pull/198/files, but some still remain)

[rgommers]

Thanks @MarcoGorelli. This LGTM overall. In particular, going from dtype: Any to dtype: DType seems nice.

My one question is about removing support for parametrizing Column by its dtype (Column[Bool] et al). Do you see that as an improvement, or a (perhaps temporary) regression?

[MarcoGorelli]

a teeny tiny regression perhaps, but I think the DType type alias would be more useful to devs, so I'd happily prioritise that one

in any case, if we did go ahead with #249 , then individual columns would become less relevant (and would probaby only be a last step before converting to ndarray anyway, or an initial step during construction)

[rgommers]

Okay, SGTM! This affects only static typing and html docs, rather than an API change. So let me hit the green button here.