use encodebytes for Python 3 to avoid DeprecationWarning

This solves the DeprecationWarning from https://github.com/boto/botocore/issues/319 which also affects us.

botocore/auth.py:453: DeprecationWarning: encodestring() is a deprecated alias, use encodebytes() return base64.encodestring(new_hmac.digest()).strip().decode('utf-8')