[mypy] fix type annotations for project euler problem188/sol1

Author: pikulet

project_euler/problem_188/sol1.py

@@ -19,7 +19,7 @@
 """
 
 
-# small helper function for modular exponentiation
+# small helper function for modular exponentiation (fast exponentiation algorithm)
 def _modexpt(base: int, exponent: int, modulo_value: int) -> int:
     """
     Returns the modular exponentiation, that is the value
@@ -36,7 +36,7 @@ def _modexpt(base: int, exponent: int, modulo_value: int) -> int:
     if exponent == 1:
         return base
     if exponent % 2 == 0:
-        x = _modexpt(base, exponent / 2, modulo_value) % modulo_value
+        x = _modexpt(base, exponent // 2, modulo_value) % modulo_value
         return (x * x) % modulo_value
     else:
         return (base * _modexpt(base, exponent - 1, modulo_value)) % modulo_value