CLN: replace %s syntax with .format in missing.py, nanops.py, ops.py
Progress towards #16130
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Replaced `%s` syntax with `.format` in `missing.py`, `nanops.py`, `ops.py`. Additionally, made some of the existing positional `.format` code more explicit.