PERF: Use is_utc check to improve performance of dateutil UTC in DatetimeIndex methods
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Benchmarks from `timeseries.py` and `timestamp.py`

```
       before           after         ratio
     [91d1c503]       [9cc84951]
-      32.6±0.7μs       6.89±0.2μs     0.21  timestamp.TimestampOps.time_normalize(tzutc())

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.

       before           after         ratio
     [91d1c503]       [9cc84951]
-      4.72±0.1μs      4.26±0.06μs     0.90  timeseries.DatetimeIndex.time_get('dst')
-      5.09±0.4ms      2.79±0.02ms     0.55  timeseries.DatetimeAccessor.time_dt_accessor_normalize(tzutc())
-        297±20μs       42.9±0.2μs     0.14  timeseries.TzLocalize.time_infer_dst(tzutc())

SOME BENCHMARKS HAVE CHANGED SIGNIFICANTLY.
```
