input: Write a python function to clean text by removing non-alphanumeric characters and converting to lowercase.
  assert clean_text('Hello, World!') == 'hello world'
  assert clean_text('Python3.8 > Python2.7') == 'python3 8 python2 7'
  assert clean_text('123 ABC def!') == '123 abc def'
output: def clean_text(text):
      import re
      cleaned_text = re.sub(r'[^a-zA-Z0-9\s]', '', text)
      return cleaned_text.lower()