input: Write a python function to maintain file line endings when rewriting files.
  assert maintain_file_line_endings("testfile.txt", u'\n') == u'\n'
  assert maintain_file_line_endings("testfile.txt", u'\r\n') == u'\r\n'
  assert maintain_file_line_endings("anotherfile.txt", u'\n') == u'\n'
output: def maintain_file_line_endings(filename, content):
      import os
      
      # Detect the existing line ending in the file
      with open(filename, 'rb') as f:
          first_line = f.readline()
          if b'\r\n' in first_line:
              existing_line_ending = '\r\n'
          elif b'\n' in first_line:
              existing_line_ending = '\n'
          elif b'\r' in first_line:
              existing_line_ending = '\r'
          else:
              existing_line_ending = os.linesep  # default system line ending
  
      # Write content to the file while maintaining the detected line ending
      with open(filename, 'w', newline=existing_line_ending) as f:
          f.write(content.replace('\n', existing_line_ending))
      
      return existing_line_ending