[Exercise: Categorical Variables] FutureWarning: Feature names only support names that are all strings

In the last code cell

```py
print("MAE from Approach 3 (One-Hot Encoding):")
print(score_dataset(OH_X_train, OH_X_valid, y_train, y_valid))
```

This shows

```
MAE from Approach 3 (One-Hot Encoding):

/opt/conda/lib/python3.7/site-packages/sklearn/utils/validation.py:1692: FutureWarning: Feature names only support names that are all strings. Got feature names with dtypes: ['int', 'str']. An error will be raised in 1.2.
FutureWarning,

17525.345719178084

/opt/conda/lib/python3.7/site-packages/sklearn/utils/validation.py:1692: FutureWarning: Feature names only support names that are all strings. Got feature names with dtypes: ['int', 'str']. An error will be raised in 1.2.
FutureWarning,
```

Adding these to step4 can fix

```py
OH_cols_train.columns = list(map(str, OH_cols_train.columns))
OH_cols_valid.columns = list(map(str, OH_cols_valid.columns))
```