python3 cannot handle hierarchical symbol tables

[Robert Haschke]

The following xacro snippet fails on python3 while it succeeds on python2:

<root version="1.0" xmlns:xacro="http://www.ros.org/wiki/xacro">
	<xacro:property name="xyz1" value="${[1,2,3]}"/>
	<xacro:property name="xyz" value="${[xyz1[i]*xyz1[i] for i in [0,1,2]]}"/>
	<a xyz="${xyz}"/>
</root>

I filed an issue on the python bug tracker: https://bugs.python.org/issue41878