Integrate with napari/pins

[jaimergp]

napari/pins will provide a subset of conda-forge curated for a painless experience with the napari plugin ecosystem. When that's ready, we need to integrate it with the plugin-manager / installer creation.

This should also cover the constraints files we would end up serving in napari.org/pins for PyPI.

[jaimergp]

Current status:

• Custom conda subchannel available at https://github.com/napari/pins/releases/download/napari-vX.Y(.Z). We need the (ugly) URL because the repodata.json stuff is heavy on weight and traffic, so GH Pages is not recommended. HTML page still available at https://napari.org/pins/conda/napari-vX.Y(.Z)/
• PyPI constraints served at https://napari.org/pins/pypi/napari-vX.Y(.Z)/pypi.txt. No HTML page yet.
• The content of these constraints is still up for debate, but the infra is there.

[jaimergp]

If this is ever implemented in the napari-plugin-manager, please take a look at #62. The idea is that we don't hardcode the pins in the code of the plugin manager, but that each napari version series has its own conda subchannel in napari/pins. The solver constraints are hence applied to the subchannel, which builds a subset of conda-forge with only packages that satisfy such constraints. The plugin manager would pick the relevant subchannel according to the host napari version, and solve plugins using only that.

That's effectively like a "server-side" constraint set. This has the advantage of not tying constraints to plugin manager releases, and can be controlled independently for all users, whether they update or not.

We didn't have time to finish the feature but the infrastructure is there and relatively straightforward to maintain.