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.
```

Hello @mroeschke! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`asv_bench/benchmarks/timeseries.py`](https://github.com/mroeschke/pandas/blob/1d1c8dbaf173842d4cdd65c016cbaba83ebe86a5/asv_bench/benchmarks/timeseries.py) !

 - There are no PEP8 issues in the file [`asv_bench/benchmarks/timestamp.py`](https://github.com/mroeschke/pandas/blob/1d1c8dbaf173842d4cdd65c016cbaba83ebe86a5/asv_bench/benchmarks/timestamp.py) !

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimes.py`](https://github.com/mroeschke/pandas/blob/1d1c8dbaf173842d4cdd65c016cbaba83ebe86a5/pandas/core/arrays/datetimes.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/datetimes.py`](https://github.com/mroeschke/pandas/blob/1d1c8dbaf173842d4cdd65c016cbaba83ebe86a5/pandas/core/indexes/datetimes.py) !

 lgtm. I bet this actually fixed a bug where a dateutil UTC value was not treated correctly? Also datetime.timezones.utc in py3 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23772?src=pr&el=h1) Report
> Merging [#23772](https://codecov.io/gh/pandas-dev/pandas/pull/23772?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/91d1c50327c209e03ffc32c5fd64f9666de81e20?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23772/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23772?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23772      +/-   ##
==========================================
- Coverage   92.24%   92.24%   -0.01%     
==========================================
  Files         161      161              
  Lines       51433    51431       -2     
==========================================
- Hits        47446    47444       -2     
  Misses       3987     3987
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.64% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.28% <14.28%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23772?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23772/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `96.2% <100%> (-0.01%)` | :arrow_down: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23772/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `98.5% <100%> (-0.01%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23772?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23772?src=pr&el=footer). Last update [91d1c50...ca60770](https://codecov.io/gh/pandas-dev/pandas/pull/23772?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @mroeschke 

as above, if you find some issues that now are closable because of the better checking (mine and @jbrockmendel comments). pls issue a PR with tests.