CI: Test editable installations

Author: effigies

.github/workflows/pythonpackage.yml

@@ -16,6 +16,7 @@ jobs:
     strategy:
       matrix:
         python-version: [3.7, 3.8]
+        pip-flags: ['', '-e']
 
     steps:
     - uses: actions/checkout@v2
@@ -26,7 +27,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install ".[dev]"
+        pip install ${{ matrix.pip-flags }} ".[dev]"
     - name: Test with pytest
       run: |
         pytest -sv --doctest-modules pydra/tasks/TODO