Default value for `skipna` should be `None` or `True` when implementing methods in new style

[PokhodenkoSA]

Are you sure that skipna=None here? It is better to refer to Pandas sources. We had am examples before where we found a bug in Pandas documentation

Originally posted by @shssf in #219

[PokhodenkoSA]

The same discussions:
#227 (review).
#227 (comment)

My explanations why skipna should be None by default:
#227 (comment)
#227 (comment)