Add DefaultSelector.fileno on platforms where it exists (#10820)

stdlib/selectors.pyi

@@ -66,8 +66,13 @@ class KqueueSelector(BaseSelector):
     def select(self, timeout: float | None = None) -> list[tuple[SelectorKey, _EventMask]]: ...
     def get_map(self) -> Mapping[FileDescriptorLike, SelectorKey]: ...
 
+# Not a real class at runtime, it is just a conditional alias to other real selectors.
+# The runtime logic is more fine-grained than a `sys.platform` check;
+# not really expressible in the stubs
 class DefaultSelector(BaseSelector):
     def register(self, fileobj: FileDescriptorLike, events: _EventMask, data: Any = None) -> SelectorKey: ...
     def unregister(self, fileobj: FileDescriptorLike) -> SelectorKey: ...
     def select(self, timeout: float | None = None) -> list[tuple[SelectorKey, _EventMask]]: ...
     def get_map(self) -> Mapping[FileDescriptorLike, SelectorKey]: ...
+    if sys.platform != "win32":
+        def fileno(self) -> int: ...

tests/stubtest_allowlists/darwin.txt

@@ -30,7 +30,6 @@ select.POLLMSG   # system dependent
 
 # Exists at runtime, but missing from stubs
 mimetypes.MimeTypes.read_windows_registry
-selectors.DefaultSelector.fileno
 
 _ctypes.dlclose
 _ctypes.dlopen

tests/stubtest_allowlists/linux.txt

@@ -15,7 +15,6 @@ socket.[A-Z0-9_]+
 
 # Exists at runtime, but missing from stubs
 mimetypes.MimeTypes.read_windows_registry
-selectors.DefaultSelector.fileno
 spwd.struct_spwd.sp_nam
 spwd.struct_spwd.sp_pwd