crecord: null out the curses attribute with `None` on failure to import

Pytype got really confused that this could be `Union[module, bool]`, and spewed
about tons of attributes that are not available on `bool`.

Differential Revision: https://phab.mercurial-scm.org/D10210

mercurial/crecord.py

@@ -64,7 +64,7 b' try:'
 
     curses.error
 except (ImportError, AttributeError):
-    curses = False
+    curses = None
 
 
 class fallbackerror(error.Abort):