BUG: Fix `skipna` default value in method signatures
Set `skipna` default values to `True` so that the value matches the
purpose of the variable and its docstring.

- [X] closes #34063
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

I did not change the [mad](https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10585) method, since its signature matches the docstring and [the implementation](https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10605). Changing that one would require a deprecation cycle I guess; if not, let me know and I will amend the commit. > I did not change the [mad](https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10585) method, since its signature matches the docstring and [the implementation](https://github.com/pandas-dev/pandas/blob/master/pandas/core/generic.py#L10605). Changing that one would require a deprecation cycle I guess; if not, let me know and I will amend the commit.

yeah this is fine looks fine. @jbrockmendel ok here? could remove the `if skipna is None` in _stat_function > could remove the if skipna is None in _stat_function

Realizing here that I may have left behind many of them in private functions, e.g.

https://github.com/pandas-dev/pandas/pull/43980/files#diff-1a2e3df0db7dd8bddc2ec4bff9de8a7a55e328e6c32e2cecde761dc9549fcd46R10390
https://github.com/pandas-dev/pandas/pull/43980/files#diff-1a2e3df0db7dd8bddc2ec4bff9de8a7a55e328e6c32e2cecde761dc9549fcd46R10443
https://github.com/pandas-dev/pandas/pull/43980/files#diff-1a2e3df0db7dd8bddc2ec4bff9de8a7a55e328e6c32e2cecde761dc9549fcd46R10509
https://github.com/pandas-dev/pandas/pull/43980/files#diff-1a2e3df0db7dd8bddc2ec4bff9de8a7a55e328e6c32e2cecde761dc9549fcd46R10553

etc. There are still 10 (+2 corresponding to the `sem` method) appearances.

Are these 10 supposed to be changed as well ? @jreback @jbrockmendel any suggestion as to what I should do concerning https://github.com/pandas-dev/pandas/pull/43980#issuecomment-941407410 ? Thanks. > @jreback @jbrockmendel any suggestion as to what I should do concerning [#43980 (comment)](https://github.com/pandas-dev/pandas/pull/43980#issuecomment-941407410) ? Thanks.

see if you break anything by changing them. IIRC we did this for some backwards compat but that was quite a while ago. > see if you break anything by changing them. IIRC we did this for some backwards compat but that was quite a while ago.

:+1: Push-forced in e15a1ce. Let's see what the CIs say. thanks @jhlegarreta very nice!