Fix compatibility with PyPy3

Author: sdispater

pendulum/datetime.py

@@ -1425,7 +1425,7 @@ def __reduce__(self):
     def __reduce_ex__(self, protocol):
         return self.__class__, self._getstate(protocol)
 
-    def _cmp(self, other):
+    def _cmp(self, other, **kwargs):
         # Fix for pypy which compares using this method
         # which would lead to infinite recursion if we didn't override
         kwargs = {

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 skipsdist = True
-envlist = py27, py35, py36, pypy
+envlist = py27, py35, py36, pypy, pypy3
 
 [testenv]
 whitelist_externals = poetry