BUG: in DataFrame.count not returning subclassed data types.
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Once #30945 is merged these tests will pass.

I also, didn't know if a PR without first submitting an issue was appropriate, but to cut down on workload I decided to give it a try. If I'm mistaken please let me know.

As for the test there are four logical branches in df.count() that needed to be address within the test:

1. Non-homogeneous data types (this is the partion of the test that is dependent on #30945 as it uses .sum()).
```
        df = tm.SubclassedDataFrame(
            {
                "Person": ["John", "Myla", "Lewis", "John", "Myla"],
                "Age": [24.0, np.nan, 21.0, 33, 26],
                "Single": [False, True, True, True, False],
            }
        )
        result = df.count()
        assert isinstance(result, tm.SubclassedSeries)
```
2. Homogeneous data.
```
        df = tm.SubclassedDataFrame({"A": [1, 0, 3], "B": [0, 5, 6], "C": [7, 8, 0]})
        result = df.count()
        assert isinstance(result, tm.SubclassedSeries)
```
3. MultiIndex with level kwarg.
```
        df = tm.SubclassedDataFrame(
            [[10, 11, 12, 13], [20, 21, 22, 23], [30, 31, 32, 33], [40, 41, 42, 43]],
            index=MultiIndex.from_tuples(
                list(zip(list("AABB"), list("cdcd"))), names=["aaa", "ccc"]
            ),
            columns=MultiIndex.from_tuples(
                list(zip(list("WWXX"), list("yzyz"))), names=["www", "yyy"]
            ),
        )
        result = df.count(level=1)
        assert isinstance(result, tm.SubclassedDataFrame)
```
4. Force length of axis to be 0. 
```
        df = tm.SubclassedDataFrame()
        result = df.count()
        assert isinstance(result, tm.SubclassedSeries)
```
Hello @EmilianoJordan! 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 2020-01-19 20:23:00 UTC you can include the other PR in this commit to get it passing.