--process-dependency-links flag has been deprecated

[ygrange]

Heya,

I really liked seeing you managed to get the installation instructions to a nicer level, but alas it has been deprecated before an alternative was around. If I understand the following discussion though there should be a solution by now:
pypa/pip#4187

[rtobar]

Hi Yan, thanks for the feedback!

This was a mechanism we used a long time ago, when we were heavily developing daliuge, and needed to modify quite a few of our dependencies to correct bugs or increase support across different python versions. The dependency list is quite stable nowadays though, and anyway PEP 508 came into place, which is what we should be based on in the future if we want to depend on a non-PyPI package.

I'll update the instruction to remove this unnecessary and now even deprecated (and since pip 19 removed) flag; in the future if we need non-PyPI packages we'll start using the PEP 508 mechanism to refer to them.