Python2とPython3のmath.round()の挙動の違い

Python2

```
>>> round(2.675, 2)
2.67
>>> round(Decimal('2.675'), 2)
2.67
```

Python3

```
>>> round(2.675, 2)
2.67
>>> round(Decimal('2.675'), 2)
Decimal('2.68')
```