release: prepare to release v0.21.0

release: prepare to release v0.21.0-rc2

Second release candidate. Might be followed by an actual release?

release: prepare to release v0.21.0-rc1

This should be followed shortly by v0.21.0, ideally with no further changes.
But since this is the first release for nearly 2 years it seems prudent to
be a little cautious and publish an actual release candidate first.

bump version to v0.20.0

Bump version to v0.19.1

Update version to v0.19.0

Preparing for v0.18.0

Release 0.17.0

Release v0.16.0

Release 0.15.0.