Add Sentinel to allowlist

Author: srittau

stdlib/@tests/stubtest_allowlists/common.txt

@@ -488,6 +488,7 @@ typing(_extensions)?\.Sized
 typing(_extensions)?\.ValuesView
 typing_extensions\.Final
 typing_extensions\.LiteralString
+typing_extensions\.Sentinal
 
 # Typing-related weirdness
 typing._SpecialForm.__call__