Bug: Resample removes timezone localization from Period.start_time
#### Code Sample

```python
from pytz import timezone
index = DatetimeIndex(['2019-01-01 06:00', '2019-01-01 07:00', '2019-01-01 08:00'], tz=timezone('Europe/Berlin'))
s = Series(index=index, data=[1, 2, 3])
print(s.index.tz)
rs = s.resample("3h", kind="period").sum()
for x in rs.index:
    print(x.start_time.tz)
    print(x.end_time.tz)
```
#### Problem description
All the Periods in the resampled PeriodIndex should have their start_time and end_time TimeStamps set with the correct Timezone.
Since the the DateTimeIndex was tz aware the attributes in the PeriodIndex should be too.
This is probably related to #13238 and #15777
#### Expected Output
Europe/Berlin
Europe/Berlin
Europe/Berlin
#### Tested Version
pandas 0.25.0