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

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

--- a/mercurial/crecord.py	Sat Mar 13 00:41:37 2021 -0500
+++ b/mercurial/crecord.py	Fri Mar 12 19:02:07 2021 -0500
@@ -64,7 +64,7 @@
 
     curses.error
 except (ImportError, AttributeError):
-    curses = False
+    curses = None
 
 
 class fallbackerror(error.Abort):