Return abbreviated implementation name.

def get_abbr_impl(env):
    """Return abbreviated implementation name."""
    impl = env.python_implementation

    if impl == "PyPy":
        return "pp"
    elif impl == "Jython":
        return "jy"
    elif impl == "IronPython":
        return "ip"
    elif impl == "CPython":
        return "cp"

    raise LookupError("Unknown Python implementation: " + impl)