🐍 Fix sys_platform Fixture Leakage breaking the CI

[gaborbernat]

The old sys_platform fixture patched sys.platform globally:

monkeypatch.setattr(sys, "platform", request.param)

With the latest virtualenv release, tox’s test suite sometimes imports virtualenv.discovery.info while this global patch is active. ⚠️ That causes other tests to see the wrong sys.platform, breaking CI.

🛠️ Solution

• Introduced a _SelectiveSys proxy (ModuleType subclass) that overrides only platform while delegating everything else to the real sys.
• Updated the fixture to patch sys only inside tox.config.loader.str_convert, leaving all other modules (including virtualenv) untouched.

✅ Benefits

• CI stability restored ✅
• Test isolation improved (no more leaking patched sys.platform into unrelated imports).
• Future-proof against similar upstream changes in virtualenv or other tools.

Resolves pypa/virtualenv#2959