Add type for distutils.command.install.SCHEME_KEYS (#4863)

Author: jdufresne

stdlib/3/distutils/command/install.pyi

@@ -1,5 +1,7 @@
 from distutils.cmd import Command
-from typing import Optional
+from typing import Optional, Tuple
+
+SCHEME_KEYS: Tuple[str, ...]
 
 class install(Command):
     user: bool