update HISTORY.rst for 1.2.0 release
update HISTORY.rst for 1.2.0 release

jd/tenacity,underrun/retrying,JioCloud/retrying,varunarya10/retrying,william-silversmith/tenacity,harlowja/retrying,rholder/retrying,bodenr/tenacity,cyrusd/retrying,muhammad-ammar/retrying