py-stem: update to 1.7.1.

The only change is a small regex fix [2] so we have forward
compatibility with tor's upcoming 0.4.0.x series. Without this Stem's
launch_tor() method will fail, timing out due to being unable to find
a bootstrap message indicating 'success'

Nyx does *not* use this method so there's no need to worry about
bumping its dependency. This only impacts applications that launch tor
such as OnionLauncher and TorNova.

(wiz)