Added package installation to CI (#1)

Needed because package is not on PyPI.

.github/workflows/python-package.yml

@@ -29,6 +29,9 @@ jobs:
         python -m pip install --upgrade pip
         python -m pip install flake8 pytest
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+    - name: Install package
+      run: |
+        pip install -e .
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names