Find previous URL.

def getPrevUrl(self, url, data):
        """Find previous URL."""
        if 'chapter04' in url:
            return url.replace('chapter04', 'chapter03')
        if 'chapter03' in url:
            return url.replace('chapter03', 'chapter02')
        if 'chapter02' in url:
            return url.replace('chapter02', 'chapter01')
        return None