Returns the path of a Pipfile in parent directories.

def find(max_depth=3):
        """Returns the path of a Pipfile in parent directories."""
        i = 0
        for c, d, f in walk_up(os.getcwd()):
            i += 1

            if i < max_depth:
                if 'Pipfile':
                    p = os.path.join(c, 'Pipfile')
                    if os.path.isfile(p):
                        return p
        raise RuntimeError('No Pipfile found!')