fix block difficulty calc frontier

Author: souradeep-das

src/ethereum/frontier/fork.py

@@ -768,7 +768,7 @@ def calculate_block_difficulty(
     # See https://github.com/ethereum/go-ethereum/pull/1588
     num_bomb_periods = (int(block_number) // 100000) - 2
     if num_bomb_periods >= 0:
-        difficulty += 2**num_bomb_periods
+        difficulty += Uint(2**num_bomb_periods)
 
     # Some clients raise the difficulty to `MINIMUM_DIFFICULTY` prior to adding
     # the bomb. This bug does not matter because the difficulty is always much