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.

misc/scripts/models-as-data/bulk_generate_mad.py

@@ -116,9 +116,7 @@ def clone_project(project: Project) -> str:
     return target_dir
 
 
-def run_in_parallel[
-    T, U
-](
+def run_in_parallel[T, U](
     func: Callable[[T], U],
     items: List[T],
     *,
@@ -516,7 +514,7 @@ if __name__ == "__main__":
         "--dca",
         type=str,
         help="Name of a DCA run that built all the projects. Can be repeated, with sources taken from all provided runs, "
-             "the last provided ones having priority",
+        "the last provided ones having priority",
         action="append",
     )
     parser.add_argument(