Missing import due to #4 regression

To fix issue #4, you deleted the code below, but the import statement should not be removed.

```python
from sklearn.preprocessing import MinMaxScaler
pd.DataFrame(  # mean of 0 and std of 1 but ranges are different (see min and max)
    MinMaxScaler().fit_transform(covid_flu[numerical_columns]),
    columns=numerical_columns
).hist()
```

I will make a pull request later.