input: Write a python function to remove non-alphabetic characters from a string.
  assert remove_non_alphabetic("Hello, World!") == "HelloWorld"
  assert remove_non_alphabetic("123 ABC def!") == "ABCdef"
  assert remove_non_alphabetic("No numbers: 123456!") == "Nonumbers"
output: def remove_non_alphabetic(s):
      return ''.join(c for c in s if c.isalpha())