Rollup merge of rust-lang#73140 - tmiasko:element-tree, r=GuillaumeGomez

Fallback to xml.etree.ElementTree

The xml.etree.cElementTree has been deprecated since Python 3.3
and removed in Python 3.9 https://bugs.python.org/issue36543.

Author: Manishearth

src/etc/htmldocck.py

@@ -114,7 +114,10 @@
     from html.parser import HTMLParser
 except ImportError:
     from HTMLParser import HTMLParser
-from xml.etree import cElementTree as ET
+try:
+    from xml.etree import cElementTree as ET
+except ImportError:
+    from xml.etree import ElementTree as ET
 
 try:
     from html.entities import name2codepoint