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 https://github.com/IntelPython/hpat/pull/219

[PokhodenkoSA]

The same discussions: https://github.com/IntelPython/hpat/pull/227#pullrequestreview-303560624. https://github.com/IntelPython/hpat/pull/227#discussion_r336238288

My explanations why skipna should be None by default: https://github.com/IntelPython/hpat/pull/227#discussion_r336326400 https://github.com/IntelPython/hpat/pull/227#discussion_r336327095