add support for keep-alive to improve performance.  This is in line with the SmartyStreets best practices (https://smartystreets.com/docs/best-practices#persist-connections)
add support for keep-alive to improve performance.  This is in line with
the SmartyStreets best practices (https://smartystreets.com/docs/best-practices#persist-connections)

audantic/smartystreets.py,bennylope/smartystreets.py