TYP fixup overload in reset_index
If we have `t.py`:
```python
import pandas as pd

inplace: bool

reveal_type(pd.DataFrame([1,2,3]).reset_index(0, inplace=True, col_fill='a'))

```

then on `master`, the last line would be revelead as `Revealed type is 'Union[pandas.core.frame.DataFrame, None]'`. With the current changes, it will be revealed as `Revealed type is 'None'`

This function takes arguments `level, drop, inplace` - if `drop` isn't passed, then `inplace` needs to be passed by keyword, but `level` can be passed positionally
Thanks Simon - merging following your approval then (also, as this is really small, and serves as an example to others doing overload issues)