Use skip_install=true for tox.ini dev environment

[hf-kklein]

so that the package itself is not installed into the site packages which is rather annoying to debug

[lord-haffi]

It would be very inconvenient if tox is not installed into the dev environment.

[hf-kklein]

It would be very inconvenient if tox is not installed into the dev environment.

is the PR title misleading? It's not skipping the installation of tox, it's about not installing the project under test in that env.

https://tox.wiki/en/latest/config.html#skip_install

Assume (in this repo) I have a src/my_project, then I hope to achieve, that with the changes from this PR, tox -e dev no longer installs my_project in the site packages although pip install .[dev] would do so.

at least that's my intention ;) I will test tomorrow if that's actually what it does.