CLN: replace usage internally of .iteritems with .items
- [x] xref #25725
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Now that Python2 is being removed, ``.iteritems`` should be deprecated, and ``.items`` should be used instead.

EDIT: This PR ended up not deprecating .iteritems, but only change its usage internally with using .items