blockchain: fix difficulty retarget

"target" is a 256 bit int, but the "bits" field in the block headers
that is used to represent target is only 32 bits.
We were checking PoW against the untruncated target value, which is a
slightly larger value than the one that can actually be represented,
and hence we would have accepted a slightly lower difficulty chain
than what the consensus requires.

electrum/blockchain.py

@@ -367,6 +367,8 @@ class Blockchain(util.PrintError):
         nActualTimespan = max(nActualTimespan, nTargetTimespan // 4)
         nActualTimespan = min(nActualTimespan, nTargetTimespan * 4)
         new_target = min(MAX_TARGET, (target * nActualTimespan) // nTargetTimespan)
+        # not any target can be represented in 32 bits:
+        new_target = self.bits_to_target(self.target_to_bits(new_target))
         return new_target
 
     def bits_to_target(self, bits: int) -> int: