Test installing project on CI

This changes the CI workflow to install the project itself to get
its dependency, rather than installing the dependency from
requirements.txt. This is the more important thing to test, because
it verifies that the project is installable and effectively
declares the dependencies it needs.

.github/workflows/pythonpackage.yml

@@ -26,10 +26,10 @@ jobs:
       with:
         python-version: ${{ matrix.python-version }}
         allow-prereleases: true
-    - name: Install dependencies
+    - name: Install project and dependencies
       run: |
         python -m pip install --upgrade pip
-        pip install -r requirements.txt
+        pip install .
     - name: Lint with flake8
       run: |
         pip install flake8