Have dataframe-api as a submodule

[honno]

Right now we manually vendor in the dataframe interchange pipeline, which originally live in https://github.com/data-apis/dataframe-api. Ideally we'd instead have that repo as a submodule, as we can effectively pin what we're working with and get updates as they come. Unfortunately the spec doesn't quite work as code due to missing imports and other minor typing problems.

So once I'm more happy with the API-as-importable-code situation, I'll submit an appropriate PR to dataframe-api repo, I should be able to drop tests/api.py in favour of a dynamically imported API.

EDIT: might be we can just wait for data-apis/dataframe-api#74 to merge

[honno]

Not a simple import as from future import __annotations__ is required for some stuff to work with py3.8/py3.9, which we want to test.