test.yml: Rely on .python-version for version

`actions/setup-python@v4` supports reading the desired Python version
from a `.python-version` file, so this way the two versions won't get
out-of-sync.

This is motived because Python 3.8.18 and 3.8.19 have different error
messages for pyright tests being run, so the Python version should match
exactly between the two.

.github/workflows/test.yml

@@ -19,7 +19,6 @@ jobs:
 
       - uses: actions/setup-python@v4
         with:
-          python-version: "3.8"
           cache: "poetry"
 
       - name: Use Node.js