CI/DOC: temporary pin environment to python 3.8
The doc build is failing on master (`/home/runner/work/pandas/pandas/doc/source/reference/api/pandas.Series.dt.rst: WARNING: document isn't included in any toctree`), and the build that starts failing seems to have switched from python 3.8 to 3.9 (I don't see any other relevant change in the environment). I suppose the default python changed at the conda(-forge) side.
Thanks @jorisvandenbossche 

I guess this may need  to be backported.

can you open an issue to unpin.
 @meeseeksdev backport 1.2.x Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 1.2.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 fd67546153ac6a5685d1c7c4d8582ed1a4c9120f
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #41092: CI/DOC: temporary pin environment to python 3.8'
```

4. Push to a named branch :

```
git push YOURFORK 1.2.x:auto-backport-of-pr-41092-on-1.2.x
```

5. Create a PR against branch 1.2.x, I would have named this PR:

> "Backport PR #41092 on branch 1.2.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                 Opened https://github.com/pandas-dev/pandas/issues/41096