Fix un-closed file when opening stop_words_file.

[RUI-LONG]

Warning detail: ResourceWarning: unclosed file <_io.BufferedReader name='D:\Anaconda\envs\know\lib\site-packages\textrank4zh\stopwords.txt'> for word in codecs.open(self.stop_words_file, 'r', 'utf-8', 'ignore'):

What I have done: Simply called close to stop_words_file.