For minor updates copy external dependencies that are not managed with pip-sync

[stitch]

So:

• move dependencies out
• pip-sync
• move dependencies in

This saves a compile step.

[stitch]

See makefile: save-custom-deps, which misses some stuff.

[baknu]

This relates, for example, to the Django update that is currently needed: https://github.com/internetstandards/Internet.nl/pull/690

[mxsasha]

I have updated all dependencies to their latest compatible version in afac9794262f61f94275f456fa9be5e2e6919a0c. I also did a bit of digging into the pip-sync situation, and I think we should not make workarounds, but rather drop pip-sync.

The purpose of pip-sync is to guarantee that a local environment is exactly per the requirements. With our complex dependencies, this purpose can already not be met. Working around it seems like we adopt a tool to then only sabotage its core function. Note that make venv already uses pip install rather than pip-sync, which I think is the right approach with our current dependency situations.

We are now up to date on dependencies so there is no rush, and we can move this over to 1.6 if needed.