cf-python currently fails for numpy 2.0.0 (release notes), released 16 Jun 2024.

We need to fix this.

I'm in favour of only supporting numpy v2. A whole bunch of our other new dependencies are for versions June 2024 or later, so I don't see the problem in only supporting numpy from that time, too.

See NCAS-CMS/cfdm#318 for a discussion.

PR to follow, after #842 has been merged.