COMPAT: Drop reference to deprecated dateutil.zoneinfo.gettz

[pganssle]

• closes COMPAT: python-dateutil deprecation warnings #19004

I'm assuming these will be handled by CI:

• tests added / passed
• 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

[pganssle]

Original PR adding this was #9123, I believe it was just a bit conservative in switching over to the preferred API.

[jreback]

thanks @pganssle yeah I think this will fix. not really sure why we had this older code.

[pganssle]

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).

[jreback]

@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]

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@@            Coverage Diff            @@
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  Files             ?      150           
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[jreback]

thanks @pganssle