__finalize__ called with other as _Concatenator?
Revisiting some 6 month old class that subclasses `Series` and overrides `__finalize__` as:

```
    def __finalize__(self, other, method=None, **kwargs):
        pd.Series.__finalize__(self, other, method=method, **kwargs)
        self.metadata = other.metadata.copy()
        self.metadata.parent = self
        return self
```

With 0.21.1 these objects can no longer be rendered:

```
>>> ser
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "pandas/core/base.py", line 80, in __repr__
    return str(self)
  File "pandas/core/base.py", line 60, in __str__
    return self.__bytes__()
  File "pandas/core/base.py", line 72, in __bytes__
    return self.__unicode__().encode(encoding, 'replace')
  File "pandas/core/series.py", line 1066, in __unicode__
    max_rows=max_rows, length=show_dimensions)
  File "pandas/core/series.py", line 1109, in to_string
    max_rows=max_rows)
  File "pandas/io/formats/format.py", line 176, in __init__
    self._chk_truncate()
  File "pandas/io/formats/format.py", line 190, in _chk_truncate
    series.iloc[-row_num:]))
  File "pandas/core/reshape/concat.py", line 213, in concat
    return op.get_result()
  File "pandas/core/reshape/concat.py", line 377, in get_result
    return cons(mgr, name=name).__finalize__(self, method='concat')
  File "pdsm/core/series.py", line 125, in __finalize__
    self.metadata = other.metadata.copy()
AttributeError: '_Concatenator' object has no attribute 'metadata'
```

I had assumed that `other` would always be of the same class as `self`.  Is `_Concatenator` a recent addition?