Add encodings.mbcs and encodings.oem (#13122)

stdlib/@tests/stubtest_allowlists/darwin.txt

@@ -32,6 +32,8 @@ select.POLLMSG   # system dependent
 _winapi
 asyncio.windows_events
 asyncio.windows_utils
+encodings.oem
+encodings.mbcs
 msvcrt
 nt
 winreg

stdlib/@tests/stubtest_allowlists/linux.txt

@@ -13,6 +13,8 @@ selectors.KqueueSelector
 _winapi
 asyncio.windows_events
 asyncio.windows_utils
+encodings.oem
+encodings.mbcs
 msvcrt
 nt
 winreg

stdlib/@tests/stubtest_allowlists/win32.txt

@@ -4,8 +4,6 @@ ctypes.GetLastError  # Is actually a pointer
 multiprocessing.reduction.AbstractReducer.DupHandle
 
 # Exists at runtime, but missing from stubs
-encodings.mbcs
-encodings.oem
 _winapi.CreateFileMapping
 _winapi.MapViewOfFile
 _winapi.OpenFileMapping