[3.14] gh-89554: Document weakref type objects as classes (GH-150678) (GH-150759)

Use the "class" directive instead of "data" for ReferenceType,
ProxyType and CallableProxyType.
(cherry picked from commit 10c4219)

Co-authored-by: Bernát Gábor <gaborjbernat@gmail.com>

Author: miss-islington

Doc/library/weakref.rst

@@ -329,17 +329,17 @@ same issues as the :meth:`WeakKeyDictionary.keyrefs` method.
    .. versionadded:: 3.4
 
 
-.. data:: ReferenceType
+.. class:: ReferenceType
 
    The type object for weak references objects.
 
 
-.. data:: ProxyType
+.. class:: ProxyType
 
    The type object for proxies of objects which are not callable.
 
 
-.. data:: CallableProxyType
+.. class:: CallableProxyType
 
    The type object for proxies of callable objects.