Update minimum PyPy version.

docs/whatsnew-4.0.rst

@@ -104,7 +104,7 @@ and also drops support for Python 3.3 so supported versions are:
 - CPython 2.7
 - CPython 3.4
 - CPython 3.5
-- PyPy 4.0 (``pypy2``)
+- PyPy 5.3 (``pypy2``)
 - PyPy 2.4 (``pypy3``)
 
 Last major version to support Python 2