input: Write a function in Python to determine if two given numbers are co-primes.
  assert is_coprime(17, 13) == True
  assert is_coprime(15, 21) == False
  assert is_coprime(25, 45) == False
output: def is_coprime(a, b):
      def gcd(x, y):
          while y:
              x, y = y, x % y
          return x
      return gcd(a, b) == 1