Fix syntax in rax_clb_nodes that breaks in Python3 (#4933)

* Use syntax that works in both Python 2 and 3 when iterating through a
    dict that's going to be mutated during iteration
  * Fixes `dictionary changed size during iteration` error
  * Fixes #4932

(cherry picked from commit 9a928d5)

changelogs/fragments/4933-fix-rax-clb-nodes.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - rax_clb_nodes - fix code to be compatible with Python 3 (https://github.com/ansible-collections/community.general/pull/4933).

plugins/modules/cloud/rackspace/rax_clb_nodes.py

@@ -252,7 +252,8 @@ def main():
                 'weight': weight,
             }
 
-            for name, value in mutable.items():
+            for name in list(mutable):
+                value = mutable[name]
                 if value is None or value == getattr(node, name):
                     mutable.pop(name)