Prepare an Altair 4.2.1 release with back-ported bug fixes and warnings

[mattijn]

This issue is on backporting bug fixes for Altair 4.2.

PR: #2827 is for the fix on jsonschema and replacing iteritems with items
PR: #2843 is about the deprecation of Vega wrappers and VegaLite 3 wrappers

This preparation is done in the v4.2.X as base branch.

I think this would be sufficient, but if there are still things missing we can add them here.

[binste]

I went through all commits in master which are not in v4.2.X and didn't find anything which would need to be added here. Looks good to me and I think it's great to get this release out. Feel free to ping me if I can help with anything.

[mattijn]

Thanks for checking @binste.

So the branch v4.2.x is publish-ready as Altair v4.2.1.

Only the release date needs to be changed in the release note for version 4.2.1 when known, see here.

Following the release notes from step 7 it can be tagged and published as version 4.2.1.

Afterwards it is probably easier to include a single commit in the https://github.com/altair-viz/altair-viz.github.io repository with the change log changes for 4.2.1, as there are no other changes in the docs.
cc: @jakevdp

[joelostblom]

This plan sounds good to me as well. I went through all the merged PRs since the last release and didn't see anything of significance other than what has been mentioned here already.

Maybe we should make a new minor release of altair_saver too to fix that selenium issue?

[mattijn]

Done per https://pypi.org/project/altair/4.2.1/