Update supported python version (#856)

Author: froggleston

_includes/install_instructions/python.html

@@ -19,8 +19,8 @@ <h3>Python</h3>
 
   <p>
     Regardless of how you choose to install it,
-    <strong>please make sure you install Python version 3.x</strong>
-    (e.g., 3.6 is fine).
+    <strong>please make sure you install a Python version >= 3.9</strong>
+    (e.g. 3.11 is fine, 3.6 is not).
   </p>
 
   {% comment %}