input: Write a python function to remove stopwords from a list of tokens.
  assert remove_stopwords(['This', 'is', 'a', 'test', 'sentence', '.']) == ['This', 'test', 'sentence', '.']
  assert remove_stopwords(['Stopwords', 'should', 'be', 'removed', 'from', 'this', 'sentence', '.']) == ['Stopwords', 'removed', 'sentence', '.']
  assert remove_stopwords(['No', 'stopwords', 'in', 'this', 'sentence', '.']) == ['No', 'stopwords', 'sentence', '.']
output: def remove_stopwords(tokens):
      stopwords = {'is', 'a', 'be', 'should', 'from', 'this', 'in'}
      return [token for token in tokens if token.lower() not in stopwords]