Enhance `test.support.os_helper.EnvironmentVarGuard` interface

[picnixz]

Feature or enhancement

Currently, we have a lot of pattern of the form:

with EnvironmentVarGuard() as env:
   if no_c in env:
       env.unset(no_c)
   if no_d in env:
       env.unset(no_d)
   for ev in {no_a, no_b}:
       if ev in env:
           env.unset(ev)

I think it makes sense to have something like this:

with EnvironmentVarGuard() as env:
   env.unset(no_a, no_b, no_c, no_d)  # unset many vars at once

or to also provide a decorator-based approach when the values to set/unset are statically known.

Features

• Allow to unset multiple environment variables: env.unset(k1, k2, ...)

Some optional features that I rejected after looking at the use cases:

• Allow to specify new environment variables at construction time (it could save a few lines when we're only modifying one or two envvars) [abandoned, not enough readable use cases]
• Add decorator-based environment guard for EnvironmentVarGuard() [abandoned for now]

I plan to create a PR today or tomorrow to illustrate the new interface.

EDIT: no need to the support for setting multiple variables at once because EnvironmentVarGuard is a mapping, so it has .update() which does the trick.

Linked PRs

• gh-131277: allow EnvironmentVarGuard to unset more than one environment variable at once #131280
• [3.13] gh-131277: allow EnvironmentVarGuard to unset more than one environment variable at once (GH-131280) #131409
• [3.12] gh-131277: allow EnvironmentVarGuard to unset more than one environment variable at once (GH-131280) #131410

See #131277 (comment) for the version labels rationale and #131280 (comment) for the backporte rationale.

[picnixz]

Ok, nvm actually __del__ checks for the presence of the variable. So we have useless checks :')

[picnixz]

Keeping the issue opened until we decide whether to backport this or not to ease future backports.

[picnixz]

Adding the version labels as I decided to backport this to ease future development. However, the API will not be publicly documented.