Add test for GH 18523 and add _tz_compare() to compare timezone of DatetimeIndex
- [x] closes #18523 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Add a new private function `_tz_compare` which compares timezones of DatetimeIndex.

@jreback 

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18596?src=pr&el=h1) Report
> Merging [#18596](https://codecov.io/gh/pandas-dev/pandas/pull/18596?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d5ffb1fc9653a47e5426121fefeccdf2be9e8c46?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18596/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18596?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18596      +/-   ##
==========================================
- Coverage   91.46%   91.41%   -0.05%     
==========================================
  Files         157      157              
  Lines       51452    51449       -3     
==========================================
- Hits        47059    47033      -26     
- Misses       4393     4416      +23
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.28% <100%> (-0.03%)` | :arrow_down: |
| #single | `40.58% <100%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18596?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.6% <100%> (-0.11%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.44% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `96.27% <0%> (-0.17%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `96.54% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.46% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.8% <0%> (+0.04%)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.11% <0%> (+0.19%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18596?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/18596?src=pr&el=footer). Last update [d5ffb1f...259d6d8](https://codecov.io/gh/pandas-dev/pandas/pull/18596?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18596?src=pr&el=h1) Report
> Merging [#18596](https://codecov.io/gh/pandas-dev/pandas/pull/18596?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d5ffb1fc9653a47e5426121fefeccdf2be9e8c46?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18596/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18596?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18596      +/-   ##
==========================================
- Coverage   91.46%   91.44%   -0.02%     
==========================================
  Files         157      157              
  Lines       51452    51449       -3     
==========================================
- Hits        47059    47048      -11     
- Misses       4393     4401       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.31% <100%> (ø)` | :arrow_up: |
| #single | `40.58% <100%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18596?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.7% <100%> (-0.02%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `96.27% <0%> (-0.17%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `96.54% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.46% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.8% <0%> (+0.04%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `81.89% <0%> (+0.19%)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/18596/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.11% <0%> (+0.19%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18596?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/18596?src=pr&el=footer). Last update [d5ffb1f...259d6d8](https://codecov.io/gh/pandas-dev/pandas/pull/18596?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > Could/should this be part of DatetimeIbdex._has_same_tz?

so looks there was an attempt to do this with ``_has_same_tz``
can leave ``DatetimeIndex._has_same_tz`` as this has a conversion for a scalar as input. But let's remove ``_timezone``, and replace ``_has_same_tz`` this call e.g. something like

```
    def _has_same_tz(self, other):

        if isinstance(other, (np.datetime64, date, datetime)):
            other = Timestamp(other)
        return timezones.tz_compare(self.tz, other.tz)
```

in tslibs/timezones.

```
cpdef tz_compare(object left, object, right):
    left = getattr(left, 'tz', '_no_tz')
    right = getattr(right, 'tz', '_no_tz')

     return get_timezone(left) == get_timezone(right)
```

and in *most* of the calls that right now are ``str(self.tz) == str(other.tz)`` can use ``_has_same_tz`` (maybe have to use ``tz_compare`` in some) xref #18558 

cc @jschendel  superseded by #19281 