Mixer should respect decimal presision on SQLAlchemy models

I have model with field defined like this:
`Column(sa.Numeric(precision=9, scale=3), nullable=False)`
and I am trying to use Mixer to put data into that model.
Sometimes it tries to insert larger values than allowed by this field which results with error:
```
sqlalchemy.exc.DataError: Mixer (): (psycopg2.errors.NumericValueOutOfRange) numeric field overflow
DETAIL: A field with precision 9, scale 3 must round to an absolute value less than 10^6.
```

Start by locating Mixer’s SQLAlchemy value-generation path and reproduce the issue with a Numeric(precision=9, scale=3) model. Done means generated values fit the declared precision and scale without causing a numeric field overflow.

Written by the indexing model from the issue text.