BUG: PeriodIndex.min/max returns int

[sinhrks]

Related to #7279. PeriodIndex.min/max should return Period ignoring NaT.

pidx = pd.PeriodIndex(['2011-01', 'NaT'], freq='M')
pidx.min()
# -9223372036854775808
pidx.max()
#492

[jreback]

is their still a need for min/max in DatetimeIndex (or were we using numpy min/max which doesn't work with NaT)?

[sinhrks]

#7279 made DatetimeIndex.min/max to handle NaT properly. This PR moves the logic to mixin to cover PeriodIndex.

[jreback]

thanks!