CLN: replace %s syntax with .format in core.computation
Progress towards #16130

- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Replaced `%s` syntax with `.format` in core.computation.  Additionally, made some of the existing positional `.format` code more explicit by modifying things like `'{0}'.format(foo)` to `'{foo}'.format(foo=foo)`.

Doesn't seem like adding tests or a whatsnew entry is relevant, and I'm not sure what a test here would look like, but I'd be happy to add either if relevant.