input: Write a Python function to concatenate two strings and return the result.
  assert concatenate_strings("Hello, ", "world!") == "Hello, world!"
  assert concatenate_strings("Python", " is awesome") == "Python is awesome"
  assert concatenate_strings("Good ", "morning") == "Good morning"
output: def concatenate_strings(str1, str2):
      return str1 + str2