input: Write a python function to clean text by removing special characters, extra whitespaces, and converting to lowercase.
  assert clean_text('Thi$$$s i$ a te$Xt!!!!!')=='this is a text '
  assert clean_text('    Hello, World!  ')=='hello world'
  assert clean_text('Python@#Programming')=='python programming'
output: def clean_text(text):
      import re
      text = re.sub(r'[^a-zA-Z0-9\s]', '', text)
      text = re.sub(r'\s+', ' ', text)
      return text.strip().lower() + ' '