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 