Update supported versions section of README

Author: horgh

README.rst

@@ -85,8 +85,9 @@ invalid IP address or an IPv6 address in an IPv4 database.
 Requirements
 ------------
 
-This code requires Python 2.7+ or 3.3+. The C extension requires CPython. The
-pure Python implementation has been tested with PyPy.
+This code requires Python 2.7+ or 3.5+. Older versions are not supported. The C
+extension requires CPython. The pure Python implementation has been tested with
+PyPy.
 
 On Python 2, the `ipaddress module <https://pypi.python.org/pypi/ipaddress>`_ is
 required.