Install package in venv

[blink1073]

We should consider "some isolation when installing the package with pip install -q -e ".[dev,test]", since there might be some dependencies in the dev or test extras that could conflict with the latest jupyter_releaser (or one of its dependencies)."

#438 (comment)

[blink1073]

I'm not sure we even need to install the python package at all in prep_git. We could try removing it, what do you think @jtpio?

[jtpio]

Removing the install of the python packages sounds good, or at least worth trying for v2.

Probably there could be some dev packages that are used to bump the version for example. But they could also be installed in one of the releaser hooks as well, for example in after-prep-git or before-bump-version.