COMPAT: Drop reference to deprecated dateutil.zoneinfo.gettz
- [X] closes #19004

I'm assuming these will be handled by CI:
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Not sure this is necessary, it's a very "behind the scenes" fix, replacing `zoneinfo.gettz` with the superior `tz.gettz()`:

- [ ] whatsnew entry

Original PR adding this was #9123, I believe it was just a bit conservative in switching over to the preferred API. thanks @pganssle yeah I think this will fix. not really sure why we had this older code. I could see someone writing code like this because they figured that `tz.gettz()` will prefer system time zones on Mac and *nix, but on Windows there *are* no system time zones, so you should use the `zoneinfo.gettz` version. That logic is already built into `tz.gettz()` though, so it's preferable to just always use `tz.gettz` (there's another way to get the equivalent of what `zoneinfo.gettz()` does if you actually care that it's dateutil's `zoneinfo` files, not the system ones). @pganssle thanks. might as well just change it, prefer the logic to exist in dateutil anyhow.

well that was the deprecation warning, so this patch should fix us. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19006?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@35443b5`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19006/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19006?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #19006   +/-   ##
=========================================
  Coverage          ?   91.57%           
=========================================
  Files             ?      150           
  Lines             ?    48937           
  Branches          ?        0           
=========================================
  Hits              ?    44815           
  Misses            ?     4122           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.94% <ø> (?)` | |
| #single | `41.75% <ø> (?)` | |


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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19006?src=pr&el=footer). Last update [35443b5...e829ff3](https://codecov.io/gh/pandas-dev/pandas/pull/19006?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @pganssle 