fix: incr to Py3.12 and cap Pooch version (#149)

There's currently a bug with the MNE sample data download we use in
`content/09_data_to_simulation/from_meg_to_hnn_notebook.ipynb`
that probably comes from an upstream issue with the Pooch library
fatiando/pooch#518
This reverses the prior commit in order to change the Python version
back to 3.12 (which is required for some Pathlib changes we use), and
also forces use of an older Pooch version to enable notebook building
successfully.

Author: asoplata

environment.yml

@@ -6,6 +6,7 @@ dependencies:
   - beautifulsoup4==4.12.3
   - pandoc==3.6.2
   - pip==25.0
+  - pooch<1.9.0
   - pypandoc==1.15
   - markdown=3.4
   - openmpi>5