CI: Moving CI dependencies to "ci/deps"
- [X] refs #23115
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@jreback you were +0 on this, do you want to take a look? Also @TomAugspurger and @jbrockmendel 