Make DTI[tz]._values and Series[tz]._values return DTA
broken off of #24024, cc @jreback @TomAugspurger 

I think the edits in core.dtypes.concat are unrelated, but they are correct regardless and easy to trim a few more lines off the diff.

The edit in tests.indexing.test_coercion was needed during troubleshooting, decided to keep it.
lgtm. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24534?src=pr&el=h1) Report
> Merging [#24534](https://codecov.io/gh/pandas-dev/pandas/pull/24534?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/091cfbb053c8522acd4f851f6a93626751e77dbb?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `44.44%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24534/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24534?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24534      +/-   ##
==========================================
+ Coverage   31.88%   31.88%   +<.01%     
==========================================
  Files         166      166              
  Lines       52427    52436       +9     
==========================================
+ Hits        16714    16721       +7     
- Misses      35713    35715       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `30.3% <44.44%> (+0.02%)` | :arrow_up: |
| #single | `31.88% <44.44%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24534?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `29.9% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `38.25% <100%> (+0.16%)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `36.75% <28.57%> (-0.2%)` | :arrow_down: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `38.04% <40%> (-0.06%)` | :arrow_down: |
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `57.57% <0%> (+0.2%)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `62.62% <0%> (+0.34%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24534?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/24534?src=pr&el=footer). Last update [091cfbb...9bcaf77](https://codecov.io/gh/pandas-dev/pandas/pull/24534?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/24534?src=pr&el=h1) Report
> Merging [#24534](https://codecov.io/gh/pandas-dev/pandas/pull/24534?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/091cfbb053c8522acd4f851f6a93626751e77dbb?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `44.44%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24534/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24534?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24534      +/-   ##
==========================================
+ Coverage   31.88%   31.88%   +<.01%     
==========================================
  Files         166      166              
  Lines       52427    52436       +9     
==========================================
+ Hits        16714    16721       +7     
- Misses      35713    35715       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `30.3% <44.44%> (+0.02%)` | :arrow_up: |
| #single | `31.88% <44.44%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24534?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `29.9% <0%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `38.25% <100%> (+0.16%)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `36.75% <28.57%> (-0.2%)` | :arrow_down: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `38.04% <40%> (-0.06%)` | :arrow_down: |
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `57.57% <0%> (+0.2%)` | :arrow_up: |
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/24534/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `62.62% <0%> (+0.34%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24534?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/24534?src=pr&el=footer). Last update [091cfbb...9bcaf77](https://codecov.io/gh/pandas-dev/pandas/pull/24534?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 gentle ping thanks @jbrockmendel 