input: Write a python function to remove non-alphabetic characters from a string.
  assert remove_non_alpha("hello!123") == "hello"
  assert remove_non_alpha("Python3.8") == "Python"
  assert remove_non_alpha("123456") == ""
output: def remove_non_alpha(input_string):
      return ''.join(char for char in input_string if char.isalpha())