input: Write a python function to convert all alphabetic characters in a string to lowercase while keeping numbers unchanged.
  assert convert_string("ABC 123 XYZ 456") == "abc 123 xyz 456"
  assert convert_string("Hello WORLD 2023") == "hello world 2023"
  assert convert_string("Python 3.9") == "python 3.9"
output: def convert_string(s):
      return ''.join(c.lower() if c.isalpha() else c for c in s)