Fix deprecation warning in SeriesGroupBy.count
See https://github.com/pandas-dev/pandas/pull/21957#issuecomment-409527005
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22155?src=pr&el=h1) Report
> Merging [#22155](https://codecov.io/gh/pandas-dev/pandas/pull/22155?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5d661c8ee06dc361ae49a63ba1e51d18e44e2255?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22155/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22155?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22155      +/-   ##
==========================================
+ Coverage   92.06%   92.06%   +<.01%     
==========================================
  Files         170      170              
  Lines       50707    50709       +2     
==========================================
+ Hits        46682    46684       +2     
  Misses       4025     4025
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.47% <100%> (ø)` | :arrow_up: |
| #single | `42.3% <50%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22155?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/22155/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9nZW5lcmljLnB5) | `86.82% <100%> (+0.03%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22155?src=pr&el=footer). Last update [5d661c8...8221d85](https://codecov.io/gh/pandas-dev/pandas/pull/22155?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @topper-123 