gh-89554: Document weakref type objects as classes

ReferenceType, ProxyType and CallableProxyType are classes (the type
objects for weak references and proxies), but were documented with the
".. data::" directive, so ":class:" cross-references to them cannot
resolve against a py:class target.

Switch these three entries to ".. class::". ProxyTypes is left as
".. data::" since it is a sequence of the proxy type objects, not a
type itself.

Author: gaborbernat

Doc/library/weakref.rst

@@ -324,17 +324,17 @@ 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.