fix python3 compatible

[kele1997]

In python3, interger divison will return a float.This will cause format error.

Python 3.7.4 (tags/v3.7.4:e09359112e, Jul  8 2019, 20:34:20) [MSC v.1916 64 bit (AMD64)] on win32
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>>> from bitmap import BitMap
>>> bm = BitMap()
>>> hex_str = "10abc"
>>> bm.fromhexstring(hex_str)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Users\kele\AppData\Local\Programs\Python\Python37\lib\site-packages\bitmap\bitmap.py", line 144, in fromhexstring
    bitstring = format(int(hexstring, 16), "0" + str(len(hexstring)/4) + "b")
ValueError: Precision not allowed in integer format specifier
>>> "0"+str(len(hex_str)/4)+"b"
'01.25b'
>>>