Install latest git-review from PyPI in quickstart
With the removal[*] of refs/publish in Gerrit 3.0.0, git-review
1.27.1 or later is needed to avoid attempting a push there and
ultimately failing. The git-review package in Ubuntu 18.04 LTS is
too old (1.26.0), so use latest from PyPI instead.
Adjust the quick-start document to install git-review with pip, and
on Debian/Ubuntu suggest including the python3-pip distro package
since it's split out separate from the interpreter packages.