ENH: support nanoseconds in Period constructor
- [x] closes #34621
- [x] closes #17053
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Picking up #34720

ASVs:
```
asv continuous -f 1.1 upstream/master HEAD -b ^period

       before           after         ratio
     [3f510600]       [2b2f1d04]
     <GH34621-nanos-for-Period^2>       <GH34621-nanos-for-Period>
+      7.46±0.2ms       11.6±0.1ms     1.55  period.PeriodIndexConstructor.time_from_ints('D', False)
+      7.57±0.3ms      11.4±0.03ms     1.51  period.PeriodIndexConstructor.time_from_ints('D', True)
+      52.8±0.6ms       59.2±0.4ms     1.12  period.PeriodIndexConstructor.time_from_ints_daily('D', True)
+      52.9±0.9ms       59.0±0.1ms     1.11  period.PeriodIndexConstructor.time_from_ints_daily('D', False)
-      3.21±0.2μs      2.90±0.02μs     0.90  period.Indexing.time_get_loc

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
PERFORMANCE DECREASED. 
```

cc @jreback https://github.com/pandas-dev/pandas/pull/34720#discussion_r491536763 in case we want this for 1.2