Ep 23: Drop python 3.8 from build matrix

[gcapes]

My requirements.txt file contains matplotlib=3.8.0, which doesn't seem to be found for python 3.8.

My failed GA run here: https://github.com/gcapes/python-intermediate-inflammation/actions/runs/6718827862
and the subsequent run with 3.8 removed here: https://github.com/gcapes/python-intermediate-inflammation/actions/runs/6718965580

The build matrix then looks like this:

strategy:
      matrix:
        os: [ubuntu-latest, macos-latest, windows-latest]
        python-version: ["3.9", "3.10"]

    runs-on: ${{ matrix.os }}

[anenadic]

This one may be a good one for you @douglowe as it is relevant to the NERC version of the course as well?

[anenadic]

Related to #274 and fixed via PR #275.