REF: make PeriodIndex.get_value wrap PeriodIndex.get_loc
AFAICT the only behavior this changes is in how datetime objects are treated.  After this they are more consistent.

```
dti = pd.date_range("2016-01-01", periods=3, freq="MS")
pi = dti.to_period("H")
ser = pd.Series(range(7, 10), index=pi)

key = dti[0]
```

master
```
>>> pi.get_loc(key)
0
>>> pi.get_value(ser, key)
KeyError: Timestamp('2016-01-01 00:00:00', freq='MS')
>>> ser.loc[key]
7
>>> ser[key]
KeyError: Timestamp('2016-01-01 00:00:00', freq='MS')
```

Under this PR, the get_value and `__getitem__` calls both return 7.