Release v0.21

[m-bock]

I noticed that there has not been a release for a while. Is there any reason for this? If not I think it would be really nice to prepare for next release.

[tchoutri]

We have a couple of blockers, like #1753

[ysangkok]

@thought2 if you want to help, it would be good to somehow diagnose this error:

#1765 (comment)

[mpilgrem]

May I ask, why was the release of servant-client-0.20.2 only a minor version change when the visible data constructor of ClientEnv was changed? I read Hackage's PVP as requiring a major version bump.

[ysangkok]

It's probably a mistake. Are you referring to the changes in #1720?

[mpilgrem]

Yes, that is correct #1720.