Update README; python 3.3 is no longer supported.

README.md

@@ -91,4 +91,4 @@ file and [list of open issues](https://github.com/dulwich/dulwich/issues).
 Supported versions of Python
 ----------------------------
 
-At the moment, Dulwich supports (and is tested on) CPython 2.7, 3.3, 3.4, 3.5, 3.6 and Pypy.
+At the moment, Dulwich supports (and is tested on) CPython 2.7, 3.4, 3.5, 3.6 and Pypy.