README: replace link from codespeak.net to RTD

The codespeak.net docs have not updated in a few years.

Author: bluetech

README.rst

@@ -16,7 +16,7 @@ execnet: distributed Python deployment and communication
 .. image:: https://img.shields.io/badge/code%20style-black-000000.svg
     :target: https://github.com/python/black
 
-.. _execnet: http://codespeak.net/execnet
+.. _execnet: https://execnet.readthedocs.io
 
 execnet_ provides carefully tested means to ad-hoc interact with Python
 interpreters across version, platform and network barriers.  It provides