Use `EnvironmentVarGuard` in tests instead of manually messing with `os.environ`

[WolframAlph]

Feature or enhancement

Proposal:

Some tests manually tweak os.environ. There is new way to do that with EnvironmentVarGuard. Identify all places and update.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-128438: Add EnvironmentVarGuard for test_urllib, test_urllib2.py and urllib2_localnet.py #128439
• gh-128438: Add EnvironmentVarGuard for test_builtin.py, test_io.py and test_locale.py #128476
• gh-128438: Add EnvironmentVarGuard for test_pdb.py #128522
• gh-128438: Use EnvironmentVarGuard for datetimetester.py to manage environment varibales #130002
• gh-128438: Use EnvironmentVarGuard for test_zoneinfo.py to manage environmen… #131870