Disallow .__call__() as workaround for non-named functions
Currently this script:
```python
import pandas as pd
funcs = [lambda: 1]
pd.eval("funcs[0]()")
```

Fails with:

    TypeError: Only named functions are supported

however this can easily be worked around by adding `.__call__`:

```python
pd.eval("funcs[0].__call__()")
```

I'm assuming we don't want to allow this workaround. This PR ensures that it will fail with the same error.