input: Write a python function to extract keywords from a given text by removing stopwords.
  assert extract_keywords("This is a sample text document that will be used to find the keywords.") == {'sample', 'text', 'document', 'used', 'find', 'keywords'}
  assert extract_keywords("It has a few sentences that talk about different topics such as sports, technology, and entertainment.") == {'sentences', 'talk', 'different', 'topics', 'sports', 'technology', 'entertainment'}
output: def extract_keywords(text):
      stopwords = {'this', 'is', 'a', 'that', 'will', 'be', 'to', 'the', 'it', 'has', 'few', 'as', 'such', 'and', 'about'}
      words = set(text.lower().split())
      keywords = words - stopwords
      return keywords