Maintenance updates (releaser v2, hatch)

[jtpio]

Perform a couple of maintenance updates:

• [x] Update config and workflows for the new releaser v2
• [x] Align the repo with the upstream cookiecutter (configuration, switch to hatch)
• [x] Require Python 3.7+

Testing on Binder: https://mybinder.org/v2/gh/jtpio/jupyterlab-plugin-playground/releaser-v2

[jtpio]

CI is now passing, also looking good on Binder.