Recommend Python 2.7 or later

We don't test with Python 2.6 any more, so likely
some things have broken; don't recommend that people
use it.

doc/installation.rst

@@ -17,7 +17,7 @@ system paths. This may require setting ``PYTHONPATH``, ``PATH`` and/or
 for which all these dependencies are available, and so is the only platform
 on which CryptoSite currently functions.
 
-* `Python <https://www.python.org>`_ 2.6 or later (Python 3 should also be OK).
+* `Python <https://www.python.org>`_ 2.7 or later (Python 3 should also be OK).
 
 * `MODELLER <https://salilab.org/modeller/>`_ plus the
   `SOAP-Protein library <https://salilab.org/SOAP/>`_.