Explicitly mention Python 3.8 support.

Author: anthony-tuininga

doc/src/index.rst

@@ -4,8 +4,8 @@ Welcome to cx_Oracle's documentation!
 
 **cx_Oracle** is a module that enables access to Oracle Database and conforms
 to the Python database API specification. This module is currently tested
-against Oracle Client 19, 18, 12, and 11.2, and Python 2.7, 3.5, 3.6 and
-3.7.
+against Oracle Client 19c, 18c, 12c, and 11.2, and Python 2.7, 3.5, 3.6, 3.7
+and 3.8.
 
 **cx_Oracle** is distributed under an open-source :ref:`license <license>`
 (the BSD license). A detailed description of cx_Oracle changes can be found in