Reintroduce __version_info__ variable

At least one application (mysql-workbench) (used to) use(s) the __version_info__
that got removed by commit 99859b8b021a5d13ca067a3e79560d6121f9c52f.

Breaking existing software with new versions of paramiko should be avoided.
This pull-request reintroduces the __version_info__ var, but fills it
from the __version__ var. No need to maintain multiple version strings.