TST: use checkstack level as per comments in groupby.agg with dicts depr testing
as per @jorisvandenbossche comments
https://github.com/pandas-dev/pandas/pull/15931#discussion_r111355878
Let's see what this gives! :-) 

(if this fails depending on the python version, we could also skip this depending on that. Although I recall that it was mainly the module deprecation rather than function deprecation that had issues depending on python version) @jorisvandenbossche seemed to work, thanks! # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15992?src=pr&el=h1) Report
> Merging [#15992](https://codecov.io/gh/pandas-dev/pandas/pull/15992?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1c4dacb4464fa0139216130b1835e5f4d4b73342?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15992/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/15992?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15992      +/-   ##
==========================================
+ Coverage      91%      91%   +<.01%     
==========================================
  Files         145      145              
  Lines       49636    49636              
==========================================
+ Hits        45170    45171       +1     
+ Misses       4466     4465       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.77% <ø> (ø)` | :arrow_up: |
| #single | `40.53% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15992?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/15992?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `95.54% <ø> (ø)` | :arrow_up: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15992?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (+0.34%)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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