hill_cipher.py: Use int() to gracefully deal with float input (#12833)

* #12821 (comment)

Author: cclauss

ciphers/hill_cipher.py

@@ -78,8 +78,10 @@ def replace_digits(self, num: int) -> str:
         'T'
         >>> hill_cipher.replace_digits(26)
         '0'
+        >>> hill_cipher.replace_digits(26.1)
+        '0'
         """
-        return self.key_string[(num)]
+        return self.key_string[int(num)]
 
     def check_determinant(self) -> None:
         """