Update versions.rst

Add a link to endoflife.date/python which has another useful visualization of Python lifecycles.

Author: warsaw

versions.rst

@@ -16,6 +16,8 @@ Python release cycle
 .. raw:: html
    :file: include/release-cycle.svg
 
+Another useful source is `endoflife.date/python <https://endoflife.date/python>`_.
+
 Supported versions
 ==================