gh-128438: Use EnvironmentVarGuard for datetimetester.py to manage environment varibales

[Wulian233]

It's easier to use this to manage environment variables

is part of #128439 (comment)

• Issue: Use EnvironmentVarGuard in tests instead of manually messing with os.environ #128438

[Wulian233]

Unrelated: I noticed that the github number just broke through 13xxxx! 😄
#130000

edit: https://github.com/python/cpython/actions/runs/13262366718/job/37021689070?pr=130002 The test failure has unlated with this

Request review @Eclips4 sorry to bother. Had a similar change before #129193. Thank you very much!

[Eclips4]

LGTM.

[pganssle]

Overall this is good, but I think the details need to be tweaked.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[Wulian233]

I have made the requested changes; please review again

Used context manager.

edit: Also need "skip news" label, label check action failed

[bedevere-app]

Thanks for making the requested changes!

@pganssle, @Eclips4: please review the changes made to this pull request.

[Wulian233]

@pganssle @Eclips4 ping~

Could we ready to merge this?

[Eclips4]

Thank you @Wulian233! Sorry for the waiting.