Fix platform-dependent type-check errors (#11345)

Use more explicit `sys.platform` checks, instead of the previous check using `in`, which mypy understands.

Fixes #11343

src/_pytest/compat.py

@@ -314,15 +314,24 @@ def safe_isclass(obj: object) -> bool:
 
 
 def get_user_id() -> int | None:
-    """Return the current user id, or None if we cannot get it reliably on the current platform."""
+    """Return the current process's real user id or None if it could not be
+    determined.
+
+    :return: The user id or None if it could not be determined.
+    """
+    # 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 == "win32" or sys.platform == "emscripten":
         # win32 does not have a getuid() function.
-    # On Emscripten, getuid() is a stub that always returns 0.
+        # Emscripten has a return 0 stub.
-    if sys.platform in ("win32", "emscripten"):
         return None
-    # getuid shouldn't fail, but cpython defines such a case.
+    else:
-    # Let's hope for the best.
+        # On other platforms, a return value of -1 is assumed to indicate that
+        # the current process's real user id could not be determined.
+        ERROR = -1
         uid = os.getuid()
-    return uid if uid != -1 else None
+        return uid if uid != ERROR else None
 
 
 # Perform exhaustiveness checking.

testing/test_parseopt.py

@@ -291,7 +291,8 @@ class TestParser:
 
 def test_argcomplete(pytester: Pytester, monkeypatch: MonkeyPatch) -> None:
     try:
-        encoding = locale.getencoding()  # New in Python 3.11, ignores utf-8 mode
+        # New in Python 3.11, ignores utf-8 mode
+        encoding = locale.getencoding()  # type: ignore[attr-defined]
     except AttributeError:
         encoding = locale.getpreferredencoding(False)
     try: