API: SparseSeries comparison now returns sparse
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Because #13985 has been fixed, `SparseSeries` comparison op now can return `SparseSeries` (it returns normal `Series` on current master). 

also fixed a bug when `SparseArray` created from `SparseSeries` may not inherit `dtype`.

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13999?src=pr) is 85.25% (diff: 100%)

> Merging [#13999](https://codecov.io/gh/pydata/pandas/pull/13999?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13999   diff @@
==========================================
  Files           139        139          
  Lines         50380      50384     +4   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42953      42955     +2   
- Misses         7427       7429     +2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [453bc26...eafc94c](https://codecov.io/gh/pydata/pandas/compare/453bc26e07ebbfea33dda9ff2f4efab683718506...eafc94c980eea9644621bc66bc3b0e2b7d6fc1ad?src=pr)
 thanks!
