Add info about working with VE and Python 3

Python 3 does not have execfile function.

docs/deploying/mod_wsgi.rst

@@ -193,5 +193,11 @@ Add the following lines to the top of your ``.wsgi`` file::
     activate_this = '/path/to/env/bin/activate_this.py'
     execfile(activate_this, dict(__file__=activate_this))
 
+For Python 3 add the following lines to the top of your ``.wsgi`` file::
+
+    activate_this = '/path/to/env/bin/activate_this.py'
+    with open(activate_this) as file_:
+        exec(file_.read(), dict(__file__=activate_this))
+
 This sets up the load paths according to the settings of the virtual
 environment.  Keep in mind that the path has to be absolute.