DEPR: in behavior for Series comparing index membership
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- [x] I have confirmed this bug exists on the latest version of pandas.

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#### Code Sample, a copy-pastable example

```python
import pandas
series = pandas.Series([1])
series.index = index=pandas.date_range("20180101", periods=1)
inOperatorOutput = 1 in series
listOperatorOutput = 1 in list(series)
assert inOperatorOutput == listOperatorOutput
```

#### Problem description
This code yields an assertion error. Because the `in` operator looks into the index of a series while the list function will take the values. 
This is not the same way how python lists and other python objects work and therefore it confuses people. I think the best solution would be to let the `in` operator take the values instead of the index.

FYI, here is an SO thread about this topic: https://stackoverflow.com/questions/51962778/python-in-operator-not-working-as-expected-when-comparing-string-and-strftime-va/51962950?noredirect=1#comment121535570_51962950