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.

.pre-commit-config.yaml

@@ -1,5 +1,7 @@
 # See https://pre-commit.com for more information
 # See https://pre-commit.com/hooks.html for more hooks
+default_language_version:
+  python: python3.12
 repos:
   - repo: https://github.com/pre-commit/pre-commit-hooks
     rev: v3.2.0