Flag to install oldest matching dependency versions

[mdekstrand]

For testing purposes, it would be useful for Flit to be able to install the oldest acceptable versions of a project's dependencies. I don't think this should extend to the oldest matching versions of transitive dependencies — that would require all transitive deps to have their minimum versions correctly specified — but for a project that is trying to maintain accurate lower bounds on dependency versions, being able to quickly install minimal acceptable dependencies would be very useful.

[takluyver]

I'm open to the idea, but note that ultimately I expect pip to handle installation again. You can already pip install . a Flit project from source. The missing piece is editable installs: pip install -e . only works with setuptools, so we have the equivalent flit install -s. But even there, Flit calls pip to install the dependencies.

So you might want to ask pip for this! I agree that there are cases where it would be useful.