I am presuming that sentiment_test.txt is just plain text, and not a specific format. You are trying to filter lines and not words. You should first tokenize and then filter the stopwords.
from nltk.tokenize import word_tokenize
from nltk.corpus import stopwords
stopset = set(stopwords.words('english'))
with open('sentiment_test.txt', 'r') as text_file:
text = text_file.read()
tokens=word_tokenize(str(text))
tokens = [w for w in tokens if not w in stopset]
print tokens
Hope this helps.