This should work, per example #6
For remote repos:
pip install -e git+https://github.com/user/project.git#egg=project[extra]
And this for local ones (thanks to @Kurt-Bourbaki):
pip install -e .[extra]
As per @Jurt-Bourbaki:
If you are using zsh you need to escape square brackets or use quotes:
pip install -e .\[extra\]
# or
pip install -e ".[extra]"