Incorrect version in trio/_version.py on v0.22.0 tag

[mgorny]

When checking out trio from the v0.22.0 tag, https://github.com/python-trio/trio/blob/v0.22.0/trio/_version.py incorrectly states the version as:

__version__ = "0.21.0+dev"

Noticed this because we use GitHub archives for packaging on Gentoo, and trio ended up being installed as 0.21.0+dev, therefore breaking version constraints in other packages.

[pquentin]

Ouch, sorry about that. I'll make an emergency v0.22.1 release.

[mgorny]

Well, it's not a big deal for us (I've just sed-ed it) but I don't know if anybody else's impacted.