bpo-36543: Revert "bpo-36543: Remove the xml.etree.cElementTree module." (GH-20117) (GH-20780)

* Revert "bpo-36543: Remove the xml.etree.cElementTree module. (GH-19108)"

This reverts commit b33e52511a59c6da7132c226b7f7489b092a33eb.
(cherry picked from commit ec88e1bca81a167e6d5c0ac635e22f84298cb1df)
diff --git a/Doc/library/xml.etree.elementtree.rst b/Doc/library/xml.etree.elementtree.rst
index 2085a85..7725e4d 100644
--- a/Doc/library/xml.etree.elementtree.rst
+++ b/Doc/library/xml.etree.elementtree.rst
@@ -15,6 +15,8 @@
 
 .. versionchanged:: 3.3
    This module will use a fast implementation whenever available.
+
+.. deprecated:: 3.3
    The :mod:`xml.etree.cElementTree` module is deprecated.