input: Write a python function to process all .txt files in the current directory and print their contents.
  assert process_txt_files() is None  # The function prints the contents, so it returns None
output: import os
  
  def process_txt_files():
      for filename in os.listdir('.'):
          if filename.endswith('.txt'):
              with open(filename, 'r') as file:
                  print(file.read())