Fix SVG histogram example.

`Element.getchildren` was removed in Python 3.9, but more importantly,
blindly picking the first subelement is completely broken since we added
metadata to SVG. Using XPath finds the `style` tag wherever it might be.

Author: QuLogic

examples/user_interfaces/svg_histogram_sgskip.py

@@ -149,7 +149,7 @@
 """ % json.dumps(hist_patches)
 
 # Add a transition effect
-css = tree.getchildren()[0][0]
+css = tree.find('.//{http://www.w3.org/2000/svg}style')
 css.text = css.text + "g {-webkit-transition:opacity 0.4s ease-out;" + \
     "-moz-transition:opacity 0.4s ease-out;}"