CI: unpin IPython
Closes https://github.com/pandas-dev/pandas/issues/30537
LGTM Does it matter this version was released two days ago? I guess IPython is technically a dev dependency so maybe not, but I would think its common enough outside of just pandas development that requiring such a new release could cause inconsistencies with other packages a user may have This is only for the doc build / dev environment. We haven't changed the requirement for users. thanks @TomAugspurger 

the less we pin in ci the better generally