Specify python<3.12 in requirements files

[fjclark]

This pull request fixes issue #147 by specifying python<3.12 in the requirements files. I've checked that I can now compile following the instructions in the README.

Sorry, I forgot to specify "ci skip" in my last commit.

• I confirm that I have merged the latest version of devel into this branch before issuing this pull request (e.g. by running git pull origin devel): y
• I confirm that I have added a changelog entry to the changelog (we will add a link to this PR as part of the review): y
• I confirm that I have permission to release this code under the GPL3 license: y

Suggested reviewers:

@chryswoods, @lohedges

[chryswoods]

Hi Finlay - we deliberately don't add the Python version as this gets in the way of us testing newer Python versions, and the compile instructions state the supported versions.

Sire itself can compile and work well with 3.12. The challenge comes from missing dependencies. It is their requirements, rather than sire's Python requirement that stops a 3.12 installation from working. As such, we don't version Python, but instead version the other dependencies (which all are > dependencies, rather than <, so that each CI/CD run will use the latest possible version, so that any incompatibility issues will be detected quickly by use during development)

[fjclark]

Hi Christopher, thanks for the explanation.