Prepare release 0.21.0

[rgommers]

No description provided.

[rgommers]

We may want to address a possible issue with the new editable install error reporting (#859 (comment)) still. Other PRs are optional; more important to get the release out soon, and we plan to do a 0.22.0 not too long after this one.

[dnicolodi]

Thanks @rgommers. I have some suggestions taking into account that the changelog

[rgommers]

updated, PTAL

[rgommers]

@dnicolodi gentle ping. Are you still thinking about non-ascii encodings (#859 (comment)) or can we go ahead and get this release out?

[dnicolodi]

Sorry for the delay, @rgommers. I have been busier than expected in the past week. The main reason why I let this PR hang is that I was hoping to get #829 in shape for this release and if it were to be merged it should have been mentioned in the changelog too. But it needs some more work IMO and I don't have time for it. On the other hand, I think the encoding issue needs to be addresses. I should be able to look at that today.

[rgommers]

I'll try revising #829, but I agree - can be in the next release, together with some of the other open PRs.

[rgommers]

With gh-887 merged, I plan to go ahead and do the release later today.