Segfault on clean-up with count example from docstrings
In https://github.com/pandas-dev/pandas/pull/19952 I have problems with a segfault when running doctests, and I could isolate at least the following code snippet that segfaults on clean-up (so only when exiting the process; an explicit del + gc.collect does not yet trigger the segfault).

```
import numpy as  np
import pandas as pd

df = pd.DataFrame({"Person": ["John", "Myla", None, "John", "Myla"],
                   "Age": [24., np.nan, 21., 33, 26],
                   "Single": [False, True, True, True, False]})
                  
res = df.set_index(["Person", "Single"]).count(level="Person")
```

