Don't upgrade system-installed version of pip

[ivan-c]

• Workaround current issue with pip >= 10

[pbugni]

I believe this was a temporary upstream problem with pip. I'd rather see us continue to use the upgraded pip unless you have reason to believe this is a bad choice, @ivan-c ?

[ivan-c]

@pbugni From this comment, it sounds like the pip installed from debian package (system version of pip) shouldn't be modified, or doesn't expect to be run as root.

I think it's against recommended best practice to run pip as root to install python modules system-wide , since python modules come from an untrusted source. This is happening isolated inside a docker image, but is still swimming upstream, so I relented since the default pip inside debian stretch docker images should be good enough (9.0.1)