[GitLabCI] Install numpy for python unittests.

.gitlab/ci/test.gitlab-ci.yml

@@ -17,6 +17,7 @@ test:ubuntu_python:
   script:
     - source venv/bin/activate
     - cd aidge_backend_cpu
+    - python3 -m pip install numpy
     - python3 -m pip list
     # Run on discovery all tests located in core/unit_tests/python and discard the stdout 
     # only to show the errors/warnings and the results of the tests