input: Write a python function to extract keywords from a text string.
  assert extract_keywords('This is a text string with some important keywords.') == ['text', 'string', 'with', 'some', 'important', 'keywords']
  assert extract_keywords('Python programming is fun and educational.') == ['python', 'programming', 'educational']
  assert extract_keywords('A quick brown fox jumps over the lazy dog.') == ['quick', 'brown', 'jumps', 'over', 'lazy']
output: def extract_keywords(text):
      import re
      from collections import Counter
  
      # Convert to lowercase
      text = text.lower()
  
      # Remove punctuation and split into words
      words = re.findall(r'\b\w+\b', text)
  
      # Filter out common stopwords
      stopwords = set([
          'i', 'me', 'my', 'myself', 'we', 'our', 'ours', 'ourselves', 'you', 'your', 'yours', 'yourself', 'yourselves',
          'he', 'him', 'his', 'himself', 'she', 'her', 'hers', 'herself', 'it', 'its', 'itself', 'they', 'them', 'their',
          'theirs', 'themselves', 'what', 'which', 'who', 'whom', 'this', 'that', 'these', 'those', 'am', 'is', 'are', 'was',
          'were', 'be', 'been', 'being', 'have', 'has', 'had', 'having', 'do', 'does', 'did', 'doing', 'a', 'an', 'the', 'and',
          'but', 'if', 'or', 'because', 'as', 'until', 'while', 'of', 'at', 'by', 'for', 'with', 'about', 'against', 'between',
          'into', 'through', 'during', 'before', 'after', 'above', 'below', 'to', 'from', 'up', 'down', 'in', 'out', 'on', 'off',
          'over', 'under', 'again', 'further', 'then', 'once', 'here', 'there', 'when', 'where', 'why', 'how', 'all', 'any',
          'both', 'each', 'few', 'more', 'most', 'other', 'some', 'such', 'no', 'nor', 'not', 'only', 'own', 'same', 'so',
          'than', 'too', 'very', 's', 't', 'can', 'will', 'just', 'don', 'should', 'now'
      ])
  
      keywords = [word for word in words if word not in stopwords]
  
      return keywords