curses: make names private that don't exist at runtime (#1308)

And add some missing return types

stdlib/3/curses/__init__.pyi

@@ -1,8 +1,6 @@
+import _curses
 from _curses import *  # noqa: F403
-# Stubgen imports a python version of has_key only if it's not present
-# in _curses (which it is in this stub)
-# from .has_key import has_key as has_key
 
-def initscr(): ...
+def initscr() -> _curses._CursesWindow: ...
 def start_color(): ...
 def wrapper(func, *args, **kwds): ...