Custom Jupyverse releaser

[davidbrochart]

Problem

Releasing Jupyverse and all its plugins is becoming too cumbersome, especially on conda-forge where one has to manually merge every PR and make sure no dependency has changed. This is mainly due to the fact that we release a new version even if there was no change. It makes it easy to install because all jupyverse* and fps* packages must have the same version, but this is not even enforced in the requirements, so one has to check manually that this is indeed the case after installing.
At the same time, plugins are independent of each other, they only respect a specification that is defined in jupyverse_api, so they should only pin jupyverse_api. And jupyverse is just a meta-package for installing a number of plugins that make up a Jupyter server.

Proposed Solution

Maybe Jupyverse should have its own releaser, since the mechanism is very specific. It would go like this:

• only release packages that have changed since the last release.
• release jupyverse_api first, then plugins.
• release jupyverse last.

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