Declare support for more recent versions of Python

I've run the tests on each of these Python versions, plus poked it
a bit manually; all seemed to work.

Note the omission of Python 3.3 -- that's sufficiently old that
I don't have an environment for it.

Author: PeterJCLaw

setup.py

@@ -84,6 +84,10 @@
     "Programming Language :: Python :: 3.0",
     "Programming Language :: Python :: 3.1",
     "Programming Language :: Python :: 3.2",
+    "Programming Language :: Python :: 3.4",
+    "Programming Language :: Python :: 3.5",
+    "Programming Language :: Python :: 3.6",
+    "Programming Language :: Python :: 3.7",
     ]
 
 if build_cext: