add ChainMap to __all__ (closes #12959)

Thanks July Tikhonov.

Lib/collections/__init__.py

 __all__ = ['deque', 'defaultdict', 'namedtuple', 'UserDict', 'UserList',
-            'UserString', 'Counter', 'OrderedDict']
+            'UserString', 'Counter', 'OrderedDict', 'ChainMap']
 
 # For backwards compatibility, continue to make the collections ABCs
 # available through the collections module.

Misc/NEWS

@@ -274,6 +274,8 @@ Core and Builtins
 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.