function for frequently repeated tz-conversion code
If cython did generators efficiently we could get rid of a ton of boilerplate where these are used.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19625?src=pr&el=h1) Report
> Merging [#19625](https://codecov.io/gh/pandas-dev/pandas/pull/19625?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6485a36483884fb817800a8380a4a4197d6df4ad?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19625/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19625?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19625      +/-   ##
==========================================
- Coverage   91.62%    91.6%   -0.03%     
==========================================
  Files         150      150              
  Lines       48798    48798              
==========================================
- Hits        44709    44699      -10     
- Misses       4089     4099      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.97% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.73% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19625?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19625/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19625/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.85% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19625?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/19625?src=pr&el=footer). Last update [6485a36...787654d](https://codecov.io/gh/pandas-dev/pandas/pull/19625?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/19625?src=pr&el=h1) Report
> Merging [#19625](https://codecov.io/gh/pandas-dev/pandas/pull/19625?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6485a36483884fb817800a8380a4a4197d6df4ad?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19625/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19625?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19625      +/-   ##
==========================================
- Coverage   91.62%    91.6%   -0.03%     
==========================================
  Files         150      150              
  Lines       48798    48798              
==========================================
- Hits        44709    44699      -10     
- Misses       4089     4099      +10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.97% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.73% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19625?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19625/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19625/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.85% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19625?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/19625?src=pr&el=footer). Last update [6485a36...787654d](https://codecov.io/gh/pandas-dev/pandas/pull/19625?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 nice!