input: Write a function to join two strings into one.
  assert join_strings("Hello, ", "world!") == "Hello, world!"
  assert join_strings("Python", " is fun") == "Python is fun"
  assert join_strings("OpenAI", " GPT-3") == "OpenAI GPT-3"
output: def join_strings(str1, str2):
      return str1 + str2