Attempt to preserve system pip doesn't work

Currently, when not using a virtualenv, bootstrap_charm_deps() function saves a copy of /usr/bin/pip3, pip upgrades the system pip, and then restores the copy of /usr/bin/pip

This is presumably trying to undo the effect of pip-upgrading pip. But it does not achieve this, as the /usr/bin/pip3 file is just an entrypoint, that uses "from pip import main", and the pip upgrade is still in
/usr/local/lib/python3.5/dist-packages, and is used by the restored /usr/bin/pip3 just fine

pip3 needs pip uninstalling, as well as restoring /usr/bin/pip3, I think