Tests .loc on sparse DataFrame #34687
- [ ✔️] closes #34687
- [ ✔️] tests added / passed

Created test case for ttps://github.com/pandas-dev/pandas/issues/34687 
Hello @weikhor! 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 2020-12-20 14:21:18 UTC @arw2019 

I have tried my test cases on my side. It work perfectly. I not sure why automatic test case still failed.  > @arw2019
> 
> I have tried my test cases on my side. It work perfectly. I not sure why automatic test case still failed.

If you look at the output it's because scipy is not being imported correctly. Take a look at what we do in other similar tests to get this to green (we won't merge if the checks don't pass) you also need to format the code (look at output from pre-commit hook) Hi @arw2019, I failed on AssertionError: ExtensionArray NA mask are different https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=50437&view=logs&j=2d7fb38a-2053-50f3-a67c-09f6e91d3121&t=449937cc-3d50-56b5-5662-e489f41f1268&l=125  and pre-commit / pre-commit (pull_request). Please assist.  > Hi @arw2019, I failed on AssertionError: ExtensionArray NA mask are different https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=50437&view=logs&j=2d7fb38a-2053-50f3-a67c-09f6e91d3121&t=449937cc-3d50-56b5-5662-e489f41f1268&l=125 and pre-commit / pre-commit (pull_request). Please assist.

Well they *are* different:
``` python
In [18]: res2 = df.loc[range(2)].loc[range(1)]
    ...: exp2 = pd.DataFrame(
    ...:             [[1.0, 0.0, np.nan, np.nan, np.nan]], dtype=pd.SparseDtype("float64", 0.0)
    ...:         )

In [19]: res2
Out[19]: 
     0    1    2    3    4
0  1.0  0.0  0.0  0.0  0.0

In [20]: exp2
Out[20]: 
     0    1   2   3   4
0  1.0  0.0 NaN NaN NaN
```
so you need to change the expected output.

Re: pre-commit, the easiest thing is to run pre-commit locally and let the hooks do the modifications @arw2019 

All checks have passed. Thank @arw2019 @jreback 
Please review and approve the pull request. Thank can you merge master and update 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. closing as stale. if you want to continue, pls ping and can re-open. > closing as stale. if you want to continue, pls ping and can re-open.

I was thinking of taking over this pull request, since it looks like it is almost finished. What would be the best way of doing that, with this living on the fork of weikhor? Just copy the changes over to a new pr? sure you can cherry pick off of the other branch is prob best