docs: update compatibility notice on main page.

docs/index.rst

@@ -406,14 +406,12 @@ a large fleet of machines, or to alert the parent of unexpected state changes.
 Compatibility
 #############
 
-The package is written using syntax compatible all the way back to **Python
-2.4** released November 2004, making it suitable for managing a fleet of
-potentially ancient corporate hardware. For example Mitogen can be used out of
-the box against Red Hat Enterprise Linux 5, released in 2007.
-
-Support for Python 3 is included using 2to3 triggered during setup.py, however
-such a Mitogen install does not support communicating with older 2.x systems. A
-future revision will support full cross 2/3 compatibility.
+Mitogen is syntax-compatible with **Python 2.4** released November 2004, making
+it suitable for managing a fleet of potentially ancient corporate hardware,
+such as Red Hat Enterprise Linux 5, released in 2007.
+
+Every combination of Python 3.x/2.x parent and child should be possible,
+however at present only Python 2.6, 2.7 and 3.6 are tested automatically.
 
 
 Zero Dependencies