BUG: correct wrong error message in df.pivot when columns=None
- [x] closes #30924 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Not at a computer to check but why is this not optional? Wouldn't not specifying it just return a Series? emm, not sure about this behaviour, but from documentation, it is supposed to return a reshaped `DataFrame`, so probably this is why this is not optional @WillAyd  OK I think I see the point here. If you can address @simonjayhawkins comments and put a note in for 1.0.1 when available I think should be good > OK I think I see the point here. If you can address @simonjayhawkins comments and put a note in for 1.0.1 when available I think should be good

@WillAyd maybe i missed the discussion, but the first patch release following 1.0.0 will be for regressions only. This should be 1.1.0? Yea makes sense. Let's do 1.1 instead (comment still applies) thanks @WillAyd and @simonjayhawkins I will add a whatsnew note once 1.1 is there ping @WillAyd @simonjayhawkins  Thanks @charlesdong1991 lgtm. @WillAyd  ping @jreback  ping @WillAyd  thanks @charlesdong1991 