fixup! blockchain: implement third fork

The `//` operator in Python was used for integer division
while it actually performs a floor division. This is only
an issue for negative numbers, e.g.

>>>  3.2 // 2
1.0
>>> -3.2 // 2
-2.0  # should be -1.0

lib/blockchain.py

@@ -304,12 +304,13 @@ def __fork_three_target(self, height, headers):
         bits = last.get('bits')
         target = Blockchain.bits2target(bits)
         first = self.get_header(last_height - 15, height, headers)
-        nActualTimespanShort = (last.get('timestamp') - first.get('timestamp')) // 15
+        nActualTimespanShort = int((last.get('timestamp') - first.get('timestamp')) / 15)
         first = self.get_header(last_height - 120, height, headers)
-        nActualTimespanMedium = (last.get('timestamp') - first.get('timestamp')) // 120
+        nActualTimespanMedium = int((last.get('timestamp') - first.get('timestamp')) / 120)
         first = self.get_header(last_height - 480, height, headers)
-        nActualTimespanLong = (last.get('timestamp') - first.get('timestamp')) // 480
+        nActualTimespanLong = int((last.get('timestamp') - first.get('timestamp')) / 480)
         nActualTimespan = (nActualTimespanShort + nActualTimespanMedium + nActualTimespanLong) // 3
+        assert nActualTimespan >= 0
         nTargetTimespan = 60
         nActualTimespan = Blockchain.__damp(nActualTimespan, nTargetTimespan)
         return Blockchain.__get_target(target, nActualTimespan, nTargetTimespan, 453, 494)