CI: fix python version

Turns out this is important for the black formatting tool to work
correctly. The formatting won't generally change between python versions
(it only depends on `black`'s version), but the formatted code needs to
be parseable by the system python version. One script uses
```python
def foo[T](x: T) -> T:
```
syntax, which is only supported in Python 3.12 and later.

Running `pre-commit` will now require a python 3.12 installation (which
is already what we mandate for internal developer environment setup).
The error in case of absence of such a version is pretty clear though.

.github/workflows/python-tooling.yml

@@ -22,6 +22,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
       - uses: pre-commit/action@646c83fcd040023954eafda54b4db0192ce70507
         name: Check that python code is properly formatted
         with: