BUG: fix non-existent variable in NDFrame.interpolate
- [x] closes #29132
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Can you add a test for the bug this fixes > Can you add a test for the bug this fixes

Absolutely! I added a test in the test_generic.TestNDFrame, is that reasonable? thanks @ellequelle  > thanks @ellequelle

👍