Update pip as part of the install process

[mikedamm]

No description provided.

[obfuscurity]

@deniszh any objections?

[deniszh]

Not sure if will work. It's better to pin pip and/or virtualenv, as in https://github.com/graphite-project/docker-graphite-statsd/blob/master/Dockerfile#L61-L62
Pip > 20.2 and virtualenv > 20, unfortunately, can cause issues - graphite-project/graphite-web#2640 (comment)

[mikedamm]

So much unversioned stuff is already getting installed by apt. This at least gets it working again on modern Ubuntu installs.