bpo-20928: support base-URL and recursive includes in etree.ElementInclude (#5723)

* bpo-20928: bring elementtree's XInclude support en-par with the implementation in lxml by adding support for recursive includes and a base-URL.

* bpo-20928: Support xincluding the same file multiple times, just not recursively.

* bpo-20928: Add 'max_depth' parameter to xinclude that limits the maximum recursion depth to 6 by default.

* Add news entry for updated ElementInclude support