input: Write a function to return a list of prime numbers up to a given number.
  assert get_primes(10) == [2, 3, 5, 7]
  assert get_primes(15) == [2, 3, 5, 7, 11, 13]
  assert get_primes(1) == []
output: def get_primes(n):
      def is_prime(num):
          if num <= 1:
              return False
          for i in range(2, int(num**0.5) + 1):
              if num % i == 0:
                  return False
          return True
  
      primes = []
      for i in range(2, n + 1):
          if is_prime(i):
              primes.append(i)
      return primes