REGR: disallow mean of period column again
This reverts parts of https://github.com/pandas-dev/pandas/pull/32426 and https://github.com/pandas-dev/pandas/pull/29941 (those are only on master, not yet released), to disallow taking the mean on a Period dtype column again. The mean for period is not supported on PeriodArray itself or for a Series of period dtype (on purpose, see discussion in https://github.com/pandas-dev/pandas/pull/24757 when mean for datetimelike was added), so for DataFrame columns it should also not work. 

See also comment at https://github.com/pandas-dev/pandas/pull/32426#issuecomment-618872954

cc @jbrockmendel 
Does this also disable median for PeriodDtype? > Does this also disable median for PeriodDtype?

Hmm, yes, indeed. 

Now, I would say that can be left out of this PR, because:

- It's actually not even tested, and only enabled on master (no regression compared to released version)
- This PR disables it in nanops, but the follow-up PR https://github.com/pandas-dev/pandas/pull/33761 to call `EA._reduce` in `frame._reduce` will make median not work as well (even if it keeps working in nanops.nanmedian), since `PeriodArray._reduce` doesn't handle it.
- Currently the support for median is also inconsistent (xref https://github.com/pandas-dev/pandas/issues/33760), since it works for DataFrame when specifying numeric_only=False, but not for Series. Which again needs `PeriodArray._reduce` to implement median

So properly supporting median basically needs `PeriodArray._reduce` to support median, and while this is certainly something we should do, my feeling is that this is better done in a dedicated PR for that (with appropriate testing). 
 @jbrockmendel does that sound OK to you? 
(and other comments on the code changes itself?) > does that sound OK to you?

Yes, this is a Worth Doing It Right situation. lgtm. this does not test / disable median? See my answer to Brock above kk great @jorisvandenbossche 
