Forcing all OS CIs to use Bash

Otherwise Windows will use PowerShell and the if-statement on the line above won't work.

.github/workflows/python-package.yml

@@ -36,6 +36,7 @@ jobs:
         python -m pip install --upgrade pip
         python -m pip install flake8 pytest-cov
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+      shell: bash
     - name: Install package
       run: |
         pip install -e .