try fix

error: unpack requires a string argument of length 4

Author: siddontang

pymysqlreplication/bitmap.py

@@ -47,6 +47,7 @@ def _createLastWordMask(self):
  used = 1 + (self._nbits - 1) & 0x7
  mask = (~(1 << used) - 1) & 0xFF
 
+ b = bytearray(0)
  l = int((self._nbits + 7) / 8) & 0x3
  if l == 1:
  self._lastWordMask = ~0 & 0xFFFFFFFF