input: Write a function to extract and return a list of verbs from a given sentence using the nltk library. The function should tokenize the sentence, tag the parts of speech, and collect all words that are verbs.
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("The dog is sleeping.")==['is', 'sleeping']
  assert extract_verbs("She was reading a book.")==['was', 'reading']
  assert extract_verbs("It will rain tomorrow.")==['will', 'rain']
  assert extract_verbs("They play football every weekend.")==['play']
output: def extract_verbs(sentence):
      from nltk import word_tokenize, pos_tag
      tokens = word_tokenize(sentence)
      tagged = pos_tag(tokens)
      verbs = [word for word, pos in tagged if pos.startswith('VB')]
      return verbs