Start with PendingDeprecationWarning

We can then document the deprecation process in docs before starting to deprecate.

Author: hugovk

setup.py

@@ -12,7 +12,7 @@
 
     warnings.warn('You are using Python {0}, which will no longer be supported in '
                   'JMESPath 1.0'.format(python_ver),
-                  DeprecationWarning)
+                  PendingDeprecationWarning)
 
 
 setup(