Update PEP 4 with xml.etree.cElementTree removal (python#1392)

See: python/cpython#12707
See: https://bugs.python.org/issue36543
Signed-off-by: Christian Heimes <[email protected]>

Author: tiran

pep-0004.txt

@@ -285,6 +285,11 @@ Deprecated modules
     Date:          2017-05-15
     Documentation: Platform is obsolete/unsupported.
 
+    Module name:   xml.etree.cElementTree
+    Rationale:     Obsolete, use xml.etree.ElementTree
+    Date:          2019-04-06
+    Documentation: Documented as deprecated since 3.3
+                   Removed in 3.9
 
 Deprecation of modules removed in Python 3.0
 ============================================