Allow list-like for `y` in `DataFrame.plot`.
In pandas 0.22 and earlier, we passing multiple values for `y` worked by accident. The code clearly assumed that `y` was a scalar, but things usually worked, perhaps with an incidental warning:

```python
In [12]: df = pd.DataFrame(np.random.uniform(size=(10, 2)), columns=['a', 'b'])

In [15]: df.plot(x='c', y=['a', 'b'])
/Users/taugspurger/miniconda3/envs/pandas-0.21.0/lib/python3.6/site-packages/pandas/plotting/_core.py:1714: UserWarning: Pandas doesn't allow columns to be created via a new attribute name - see https://pandas.pydata.org/pandas-docs/stable/indexing.html#attribute-access
  series.name = label
Out[15]: <matplotlib.axes._subplots.AxesSubplot at 0x10f83b198>
```

On master, this will currently raise. I think that we should explicitly support list-likes for `y`.

Tagging for 0.23 since I think this should be a blocker.

xref https://github.com/pandas-dev/pandas/pull/18695 where we made the change.