Install Python package as editable for mybinder.org

[rgaiacs]

When configuring a Git repository for https://mybinder.org/, the recommended way is to use a conda environment, see https://repo2docker.readthedocs.io/en/latest/configuration/research/#environment-yml-install-a-conda-environment. If you are not using conda, the recommended way is to use a requirements.txt. For Python packages, a setup.py is supported. But setup.py has been replaced by pyproject.toml and https://mybinder.org/ does not have support for pyproject.toml, see jupyterhub/repo2docker#1444.

This pull request provides a work around to install the package using the existing pyproject.toml. All the dependencies listed in pyproject.toml are installed.