input: Write a python function to filter strings that only contain alphabetic characters.
  assert filter_strings(['Hello', 'World', '123', 'Python3', 'is', 'awesome!']) == ['Hello', 'World', 'is']
  assert filter_strings(['abc', '123', 'def', '456', 'ghi']) == ['abc', 'def', 'ghi']
  assert filter_strings(['Only', 'Alpha', '123Numbers', 'Symbols!']) == ['Only', 'Alpha']
output: def filter_strings(strings):
      return [s for s in strings if s.isalpha()]