add ChainMap to __all__ (closes #12959)

Thanks July Tikhonov.
diff --git a/Misc/NEWS b/Misc/NEWS
index ef12cfd..d94f33c 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -274,6 +274,8 @@
 Library
 -------
 
+- Issue #12959: Add collections.ChainMap to collections.__all__.
+
 - Issue #12567: Add curses.unget_wch() function. Push a character so the next
   get_wch() will return it.