gh-128438: Add EnvironmentVarGuard for test_builtin.py, test_io.py and test_locale.py

[WolframAlph]

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

[picnixz]

@vstinner: I plan to merge this with the following title:

Use `EnvironmentVarGuard` for `test_{builtin,io,locale}.py`

And the following commit message:

Modifying locale-related environment variables in `Lib/test/test_builtin.py`,
`Lib/test/test_io.py` and `Lib/test/test_locale.py` is now achieved by using
an `EnvironmentVarGuard` context instead of an explicit `try-finally` block.

[vstinner]

LGTM