Format key, value pairs from attrs into graphviz attrs format

    Examples
    --------
    >>> format_attrs({'key1': 'value1', 'key2': 'value2'})  # doctest: +SKIP
    '[key1=value1, key2=value2]'

def format_attrs(attrs):
    """
    Format key, value pairs from attrs into graphviz attrs format

    Examples
    --------
    >>> format_attrs({'key1': 'value1', 'key2': 'value2'})  # doctest: +SKIP
    '[key1=value1, key2=value2]'
    """
    if not attrs:
        return ''
    entries = ['='.join((key, value)) for key, value in iteritems(attrs)]
    return '[' + ', '.join(entries) + ']'