slightly modified one test so it's less MySQL specific

I was getting the following warning:
```
SADeprecationWarning: Passing arguments to type object constructor <class 'sqlalchemy.types.BigInteger'> is deprecated
  id = Column(BigInteger(unsigned=True), default=text("uuid_short()"), primary_key=True, autoincrement=False)
```

The issue can be fixed by importing from `sqlalchemy.dialect.mysql` but I thought it'd be better to have things less specific to a particular DB backend.  I'm hoping `round()` is a function available in everything supported by SQLAlchemy!