Revert sys.platform verification to simple comparison (#11998)

As the comment above it states, we need to keep the comparison simple so mypy can understand it, otherwise it will fail to properly handle this on Windows and will flag ``os.getuid()`` missing.

src/_pytest/compat.py

@@ -289,7 +289,7 @@ def get_user_id() -> int | None:
     # mypy follows the version and platform checking expectation of PEP 484:
     # https://mypy.readthedocs.io/en/stable/common_issues.html?highlight=platform#python-version-and-system-platform-checks
     # Containment checks are too complex for mypy v1.5.0 and cause failure.
-    if sys.platform in {"win32", "emscripten"}:
+    if sys.platform == "win32" or sys.platform == "emscripten":  # noqa: PLR1714
         # win32 does not have a getuid() function.
         # Emscripten has a return 0 stub.
         return None