Pin Python dependencies and install them in a virtualenv

[damz]

(Alternative version from #7403)

Installing sphinx started to fail about a week ago, with pip incorrectly trying to remove a Python package shipped at the system level in Platform.sh environment.

It looks like it started failing because of a bug in the older version of pip that ships with Debian Jessie. We (Platform.sh) install global Python dependencies as user-site packages (using pip --user). We also ship system-site packages that we use for our own benefit.

So two issues here:

• First, that older version of pip. We are going to look into upgrading it (a new batch of images based on Debian Stretch are cooking);
• The build dependencies of the Symfony Docs are not pinned, which makes the build non-repeatable. I'm guessing that in this case it broke when jinja2 ended up requiring markupsafe>=0.23 (but that's just a guess).

Fix the issue for good by installing in a virtualenv and pinning all the dependencies via pip freeze.

[javiereguiluz]

Amazing! @damz thanks so much for looking into this. Now I'm sure I would have never fixed it 😅

[damz]

You can pick either this one or #7403, and it is purely a "pick your poison" question:

• This one you will have to update the pinned dependencies from time to time, but the build is repeatable;
• Move python dependencies inside a virtualenv #7403 you don't have to update the dependencies (you get updated versions automatically), but that might make the build randomly fail.

[javiereguiluz]

We've finally decided to go for this version, so we'll close #7403 and we'll merge this soon. @damz thanks for your great help solving this issue!

[xabbuh]

Thank you so much for fixing our platform.sh builds @damz! Happy to merge it.