DOC: fix flake8 issue in groupby.rst
Fixed flake8 issue in groupby.rst, see Issue #24178 for more info.

The following flake8 'errors' have been fixed:
- doc/source/groupby.rst:72:18: F821 undefined name 'obj'
- doc/source/groupby.rst:72:30: F821 undefined name 'key'
- doc/source/groupby.rst:73:18: F821 undefined name 'obj'
- doc/source/groupby.rst:73:30: F821 undefined name 'key'
- doc/source/groupby.rst:74:18: F821 undefined name 'obj'
- doc/source/groupby.rst:74:31: F821 undefined name 'key1'
- doc/source/groupby.rst:74:37: F821 undefined name 'key2'
- doc/source/groupby.rst:1306:42: F821 undefined name 'report_func'
- doc/source/groupby.rst:242:15: E225 missing whitespace around operator
- doc/source/groupby.rst:242:15: E999 SyntaxError: invalid syntax
- doc/source/groupby.rst:242:19: E225 missing whitespace around operator

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24363?src=pr&el=h1) Report
> Merging [#24363](https://codecov.io/gh/pandas-dev/pandas/pull/24363?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/88aebed582f5ae898aa78e8b3491c212e92c5d13?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24363/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24363?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24363   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files         162      162           
  Lines       51808    51808           
=======================================
  Hits        47817    47817           
  Misses       3991     3991
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (ø)` | :arrow_up: |
| #single | `42.99% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24363?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/24363?src=pr&el=footer). Last update [88aebed...dec7e63](https://codecov.io/gh/pandas-dev/pandas/pull/24363?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/24363?src=pr&el=h1) Report
> Merging [#24363](https://codecov.io/gh/pandas-dev/pandas/pull/24363?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/08c920eab602dac261b8fe55ffe439593c095e12?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/24363/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24363?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24363      +/-   ##
==========================================
+ Coverage    92.3%   92.31%   +<.01%     
==========================================
  Files         163      163              
  Lines       51987    51987              
==========================================
+ Hits        47989    47990       +1     
+ Misses       3998     3997       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.72% <ø> (ø)` | :arrow_up: |
| #single | `42.99% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24363?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24363/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.84% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24363?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/24363?src=pr&el=footer). Last update [08c920e...84c0110](https://codecov.io/gh/pandas-dev/pandas/pull/24363?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @LJArendse do you have time to address comments of the reviews? @datapythonista Hi, yes I do. Sorry for the 'radio silence'. thanks @LJArendse 