Series with PostalCode logical type can have `float` or `str` elements.

Series with PostalCode logical type can have `float` or `str` elements.

For example,
```python
ser = pd.Series([12345, 67890]).astype('category')
ser = ww.init_series(ser, logical_type='PostalCode')
```
In the above code block, the elements of the series are floats, but in the following, they are strings:

```python
ser = pd.Series(["12345", "67890"]).astype('category')
ser = ww.init_series(ser, logical_type='PostalCode')
```

Both are valid initializations. We should decide whether we want to support both data types for the PostalCode logical type.

This issue was discussed here. https://github.com/alteryx/featuretools/pull/2365