You don't always want to install the actual package necessarily (e.g. dev) - using requirements separately lets you install the deps but not the package...

What if you have your own package index? You can use `--index-url` in requirements.txt to specify that.

You can use `--index-url` when calling pip: https://pip.pypa.io/en/stable/reference/pip_wheel/#index-url

You can also use `dependency_links` in your setup.py to specify this, which allows deps on github etc.