def blacken(c):
    if not PYTHON.startswith("3.6"):
        msg = "Not blackening, since Python {} != Python 3.6".format(PYTHON)
        print(msg, file=sys.stderr)
        return
    config = c.config.get("travis", {}).get("black", {})
    version = config.get("version", "18.5b0")
    c.run("pip install black=={}".format(version))
    checks.blacken(c, check=True, diff=True)