Fix dependency resolution for six
Fix dependency resolution for six

pip isn't smart enough to install a version of six that satisfies both
purplex and robobrowser, so we need to specify it manually.

das7pad/hangups,taigrr/hangups,tdryer/hangups,j16sdiz/hangups,jodizzle/hangups,ddboline/hangups,dxd132630/hangups,hangoutsbot/hangups,AshenDrops/hangups,baskervald/hangups