input: Write a Python function to concatenate two strings with a space in between.
  assert concatenate_strings("I am writing a Python program.", "It is exciting to learn something new.") == "I am writing a Python program. It is exciting to learn something new."
  assert concatenate_strings("Hello", "World") == "Hello World"
  assert concatenate_strings("Good", "Morning") == "Good Morning"
output: def concatenate_strings(str1, str2):
      return str1 + " " + str2