Add method to handle mixed string and non-string types in object column.
#### Problem description

Since the version 1.0.0 and the creation of the StringDtype the use of an object column to handle string would decrease. Nevertheless, and [as in the new documentation is stated](https://pandas.pydata.org/pandas-docs/stable/whatsnew/v1.0.0.html)
```
There isn’t a clear way to select just text while excluding non-text, but still object-dtype columns.
```
The way how I had handle this issue in the past have changed throught the time and depending on my needs. I think this is a great opportunity to define a way to proceed in this cases. I dont know the feelings of the devs on this issue. In any case, here we go my two cents to have something to start a discussion about this:

- Series.select_dtypes() : Select subset of rows based on the type. maybe with an option to convert non matching rows to NA instead of dropping it.