Update to CMake 3.21.3

[ihnorton]

No description provided.

[jeroen]

Thanks!

[jeroen]

Btw, is there some feature that requires this version?

[ihnorton]

We are considering bumping the minimum required cmake version to 3.21. There's no single feature, but these changes and others in the last few versions add up to a lot of usability improvements. The consideration is basically that if we are going to bump to 3.18, practically we might as well go to 3.21 b/c someone is always going to need to update. So I'm testing that out in our CI.

There are well-maintained cmake binaries on pypi now so IMO pinning to some ubuntu/debian LTS version isn't really necessary anymore (those wheels are built with manylinux2010 so they are compatible back to CentOS 6).