Remove python 3.9 tests as they crash for now

The error traceback:

/opt/hostedtoolcache/Python/3.9.12/x64/lib/python3.9/site-packages/discrete_optimization/tsp/tsp_model.py:267: in <module>
    class Point2D(Point, NamedTuple):
/opt/hostedtoolcache/Python/3.9.12/x64/lib/python3.9/typing.py:1929: in _namedtuple_mro_entries
    raise TypeError("Multiple inheritance with NamedTuple is not supported")
E   TypeError: Multiple inheritance with NamedTuple is not supported

Coming from python/cpython#19363

Potential solution: replace NamedTuple by Dataclass(frozen=True)

Author: nhuet

.github/workflows/build_do.yml

@@ -33,7 +33,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.7", "3.8", "3.9"]
+        python-version: ["3.7", "3.8"]
     env:
       MINIZINC_URL: https://github.com/MiniZinc/MiniZincIDE/releases/download/2.6.3/MiniZincIDE-2.6.3-x86_64.AppImage
     steps: