We have support for installing python from pyenv, but it currently
requires setting the python version explicitly as an argument. If
the repo in question has a .python-version file, we shouldn't need to
require the user to provide that version a second time. Instead, we can
read from the file for the install step.
Change-Id: Ic4c2d3fc7f55169cec5211010fc3a9622fa324d1
7a58814cda