Error handler for `shutil.rmtree`.

    If the error is due to an access error (read-only file), it
    attempts to add write permission and then retries.
    If the error is for another reason, it re-raises the error.

    Usage: `shutil.rmtree(path, onerror=onerror)

def onerror(function, path, excinfo):
    """Error handler for `shutil.rmtree`.

    If the error is due to an access error (read-only file), it
    attempts to add write permission and then retries.
    If the error is for another reason, it re-raises the error.

    Usage: `shutil.rmtree(path, onerror=onerror)"""
    if not os.access(path, os.W_OK):
        # Is the error an access error?
        os.chmod(path, stat.S_IWUSR)
        function(path)
    else:
        raise