fix groupby.mean for only numeric values

[jajcayn]

There was a change in the default behaviour for groupby(...).mean(), now the default is numeric_only=False

[raphaelvallat]

Awesome thanks for catching this! A few comments:

1. We need to bump the pandas dependency to 1.1, since numeric_only is not present in 1.0
2. I think there are many other groupby().mean() in the Pingouin code. To be safe I think we need to edit all of them.
3. Unit tests are failing for unrelated reasons. I'll fix later.

I'll work on 1 and 2 now. Thanks