Fix test_unstack
The test was failing if the ExtensionDtype had an na_value that wasn't equivalent to nan

- [x] closes #36986 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @jrm5100! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-10-08 19:24:05 UTC I restarted the failed numpy dev build (timed out) thanks @jrm5100  > Thanks, I guess it came down to this passing
> 
> ```python
> In [45]: a = pd.Series([1, np.nan], dtype=object)
> 
> In [46]: b = pd.Series([1, pd.NA], dtype=object)
> 
> In [47]: pd.testing.assert_series_equal(a, b)
> ```
> 
> FYI, we're making a list of 3rd-party extension types at https://pandas.pydata.org/pandas-docs/stable/ecosystem.html#extension-data-types, if you want to add pandas-genomics to ecosystem.rst

shouldn't this raise as these are *not* equal > shouldn't this raise as these are not equal

I think so. Looks like it's come up in https://github.com/pandas-dev/pandas/issues/18463 / https://github.com/pandas-dev/pandas/issues/27613