Build the docs.

def docs(session):
    """Build the docs."""

    # Build docs against the latest version of Python, because we can.
    session.interpreter = 'python3.6'

    # Set the virtualenv dirname.
    session.virtualenv_dirname = 'docs'

    # Install Sphinx and also all of the google-cloud-* packages.
    session.chdir(os.path.realpath(os.path.dirname(__file__)))
    session.install('-r', os.path.join('docs', 'requirements.txt'))

    # Build the docs!
    session.run(
        'bash', os.path.join('.', 'scripts', 'update_docs.sh'))