Update versions tested by tox

Author: petertodd

README.md

@@ -102,14 +102,6 @@ system, you can run unit tests for multiple Python versions:
 
     ./runtests.sh
 
-Currently, the following implementations are tried (any not installed are
-skipped):
-
-    * CPython 3.3
-    * CPython 3.4
-    * CPython 3.5
-    * PyPy3
-
 HTML coverage reports can then be found in the htmlcov/ subdirectory.
 
 ## Documentation

tox.ini

@@ -2,7 +2,7 @@
 
 [tox] #-------------------------------------------------------------------
 
-envlist = reset,py33,py34,py35,pypy3,stats
+envlist = reset,py33,py34,py35,py36,py37,pypy3,stats
 skip_missing_interpreters = True
 
 [testenv] #---------------------------------------------------------------