Minor improvements to CI workflows

[AlexWaygood]

• Use the new allow-prereleases key for actions/setup-python. If we set this to true, then we can just use 3.12 in our CI matrix, instead of 3.12-dev. That means we won't have to worry about updating the matrix from 3.12-dev to 3.12 when 3.12.0 comes out in the autumn; our CI will automatically start using the latest version of Python 3.12.
• Colorise the output of flake8.
• Don't bother upgrading pip in CI. It's not really necessary, and wastes a few seconds. Disable the "you're using an older version of pip" message instead, using the PIP_DISABLE_PIP_VERSION_CHECK environment variable.

[AlexWaygood]

Docs on allow-prereleases: https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#allow-pre-releases