Setting xticks with time series broken in 0.21
Hey. So this worked in 0.20 but not in 0.21:

```python
import matplotlib.pyplot as plt
xticks = pd.date_range(start="10/10/2017", end="11/11/2017",
                       freq='D')
plt.xticks(xticks, xticks.strftime("%a %m-%d"))
```
> TypeError: Cannot compare type 'Timestamp' with type 'float'

I'm not sure that was on purpose. So far every ~minor~ major release of pandas since my book came out broke the code in my book :-/ It would be nice to get deprecation warnings.
