Use feature detection instead of version detection (#846)

https://docs.python.org/3/howto/pyporting.html#use-feature-detection-instead-of-version-detection

gym/error.py

@@ -90,12 +90,11 @@ def __unicode__(self):
         else:
             return self._message
 
-    if sys.version_info > (3, 0):
-        def __str__(self):
-            return self.__unicode__()
-    else:
-        def __str__(self):
+    def __str__(self):
+        try:               # Python 2
             return unicode(self).encode('utf-8')
+        except NameError:  # Python 3
+            return self.__unicode__()
 
 
 class APIConnectionError(APIError):