STY: use pytest.raises context syntax (groupby)
xref #24332
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=h1) Report
> Merging [#24828](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/08f92c4c86fbb4e03d030f24ea66402f6deee6df?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24828/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24828   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52383    52383           
=======================================
  Hits        48396    48396           
  Misses       3987     3987
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <100%> (ø)` | :arrow_up: |
| #single | `42.91% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/24828/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `96.8% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=footer). Last update [08f92c4...f2fab1d](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=h1) Report
> Merging [#24828](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/08f92c4c86fbb4e03d030f24ea66402f6deee6df?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24828/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24828      +/-   ##
==========================================
- Coverage   92.38%   92.38%   -0.01%     
==========================================
  Files         166      166              
  Lines       52383    52383              
==========================================
- Hits        48396    48395       -1     
- Misses       3987     3988       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <100%> (ø)` | :arrow_up: |
| #single | `42.91% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/24828/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGJ5LnB5) | `96.8% <100%> (ø)` | :arrow_up: |
| [pandas/core/groupby/grouper.py](https://codecov.io/gh/pandas-dev/pandas/pull/24828/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS9ncm91cGVyLnB5) | `98.16% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24828/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.04% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=footer). Last update [08f92c4...f2fab1d](https://codecov.io/gh/pandas-dev/pandas/pull/24828?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks for the review @WillAyd. have made changes as requested.  lgtm. @WillAyd merge when ok. Thanks @simonjayhawkins 