WIP: groupby skipna
- [ ] closes #15675
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry


New duplicate of #15772 since I had lost the previous branch. Work in progress.

Hello @mayukh18! Thanks for opening this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:

* In the file [`pandas/core/groupby/generic.py`](https://github.com/pandas-dev/pandas/blob/7a74e3427da0c1368ae91112506afc0bbca67130/pandas/core/groupby/generic.py):

> [Line 349:89](https://github.com/pandas-dev/pandas/blob/7a74e3427da0c1368ae91112506afc0bbca67130/pandas/core/groupby/generic.py#L349): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (101 > 88 characters)
> [Line 365:89](https://github.com/pandas-dev/pandas/blob/7a74e3427da0c1368ae91112506afc0bbca67130/pandas/core/groupby/generic.py#L365): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (89 > 88 characters)

* In the file [`pandas/core/groupby/groupby.py`](https://github.com/pandas-dev/pandas/blob/7a74e3427da0c1368ae91112506afc0bbca67130/pandas/core/groupby/groupby.py):

> [Line 1303:89](https://github.com/pandas-dev/pandas/blob/7a74e3427da0c1368ae91112506afc0bbca67130/pandas/core/groupby/groupby.py#L1303): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (102 > 88 characters)
> [Line 1716:89](https://github.com/pandas-dev/pandas/blob/7a74e3427da0c1368ae91112506afc0bbca67130/pandas/core/groupby/groupby.py#L1716): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (103 > 88 characters)

* In the file [`pandas/core/groupby/ops.py`](https://github.com/pandas-dev/pandas/blob/7a74e3427da0c1368ae91112506afc0bbca67130/pandas/core/groupby/ops.py):

> [Line 717:9](https://github.com/pandas-dev/pandas/blob/7a74e3427da0c1368ae91112506afc0bbca67130/pandas/core/groupby/ops.py#L717): [E265](https://duckduckgo.com/?q=pep8%20E265) block comment should start with '# '
> [Line 731:9](https://github.com/pandas-dev/pandas/blob/7a74e3427da0c1368ae91112506afc0bbca67130/pandas/core/groupby/ops.py#L731): [E265](https://duckduckgo.com/?q=pep8%20E265) block comment should start with '# '
> [Line 809:17](https://github.com/pandas-dev/pandas/blob/7a74e3427da0c1368ae91112506afc0bbca67130/pandas/core/groupby/ops.py#L809): [E265](https://duckduckgo.com/?q=pep8%20E265) block comment should start with '# '

 This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. Thanks for the PR, but while this is a WIP PR it appears further development has gone stale. For drafts we suggest marking as such in the Github UI in the future. Closing but happy to reopen if interested in further developement cc: @mzeitlin11 

I'm interested in getting `skipna` to work on `groupby` (see #39983 for my use now that `level` is no longer a parameter for `sum`, and #15675 for the general issue), but I'm not very familiar with Cython. I can definitely take the initial PR as is and take care of linting issues and whatsnew, but could use some guidance on what kind of tests would be necessary. > cc: @mzeitlin11
> 
> I'm interested in getting `skipna` to work on `groupby` (see #39983 for my use now that `level` is no longer a parameter for `sum`, and #15675 for the general issue), but I'm not very familiar with Cython. I can definitely take the initial PR as is and take care of linting issues and whatsnew, but could use some guidance on what kind of tests would be necessary.

Sounds great, that would be awesome! If you can put up a new pr with what you mention above and ping me, I'd be happy to help out with any cython / discussion of tests