test: nbmake should use py39 kernel name

python -m ipykernel install --user --name py39

Author: chilango74

.travis.yml

@@ -8,8 +8,10 @@ install:
   - pip install -r requirements_test.txt
   - pip install pytest-cov
   - pip install coveralls
+  - pip install ipykernel
 # command to run tests
 script:
+  - python -m ipykernel install --user --name py39
   - pytest --nbmake examples
   - pytest tests --cov=okama/
 after_success: