在python2.7的doc中，round()的最后写着，
"Values are rounded to the closest multiple of 10 to the power minus ndigits; if two multiples are equally close, rounding is done away from 0."
保留值将保留到离上一位更近的一端（四舍六入），如果距离两端一样远，则保留到离0远的一边。所以round(0.5)会近似到1，而round(-0.5)会近似到-1。
但是到了python3.5的doc中，文档变成了
"values are rounded to the closest multiple of 10 to the power minus ndigits; if two multiples are equally close, rounding is done toward the even choice."
如果距离两边一样远，会保留到偶数的一边。比如round(0.5)和round(-0.5)都会保留到0，而round(1.5)会保留到2。
Python 2.7 中的结果如下：
>>> round(-0.5)
-1.0
>>> round(0.5)
1.0
>>> round(1.5)
2.0
>>> round(2.5)
3.0
>>> round(3.5)
4.0
>>> round(4.5)
5.0
>>> round(6.5)
7.0
Python 3.8 中的结果如下：
round(-0.5)
0
round(0.5)
0
round(1.5)
2
round(2.5)
2
round(3.5)
4
round(4.5)
4
round(5.5)
6
round(6.5)
6