Add setuptools to requirements.txt

[cristeigabriel]

Running setup.py install requires the presence of the setuptools pip package. Along with a fresh Python install, adding this entry to requirements.txt makes following the installation documentation work without trouble.

As an addendum, I would recommend checking if the Python version is in WindowsApps in the setup.py file, in which case to bail early. I've found that SetupTools wants to do modifications in the Python path, which it wouldn't have permission to the way the application is packaged. If welcomed, I would be willing to also implement this to make our PR more substantial.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.