QST: Behaviour of `mean()` for a Series of strings.
Greetings, Pandas devs! My question is about the behaviour of `mean()` for a Series of strings:

```
In [1]: import pandas as pd

In [2]: pd.Series(['1', '2']).mean()
Out[2]: 6.0
```

From https://github.com/pandas-dev/pandas/issues/34671#issuecomment-641927667, the result is `6` because:

> "1" + "2" is "12", and then we convert to numeric 12, and divide by the count

This feels like somewhat implicit behaviour that makes assumptions about how the mean is computed.

Would it be more desirable to throw here? Or perhaps return `1.5` instead?
