Utilities/Sphinx: Fix cmake domain document removal with python3

In the domain clear_doc method, avoid removing entries from a dictionary
while iterating over it.  Instead accumulate a set of entries to remove
at the end.

Utilities/Sphinx/cmake.py

@@ -290,8 +290,11 @@ class CMakeDomain(Domain):
     }
 
     def clear_doc(self, docname):
+        to_clear = set()
         for fullname, (fn, _) in self.data['objects'].items():
             if fn == docname:
+                to_clear.add(fullname)
+        for fullname in to_clear:
             del self.data['objects'][fullname]
 
     def resolve_xref(self, env, fromdocname, builder,