Remove reference to requirements.txt in action

Should use the requirements specified in pyproject.toml

.github/workflows/pythonpackage.yml

@@ -29,7 +29,7 @@ jobs:
         python -m pip install --upgrade pip 
         pip install setuptools wheel
         pip install numpy==${{ matrix.numpy-version }} cython
-        pip install -r requirements.txt
+        python -m pip install .
     - name: Lint with flake8
       run: |
         pip install flake8