to build pyfftw from source for 3.9 we'll need it installed on CI

Author: garrettwrong

.github/workflows/workflow.yml

@@ -15,6 +15,7 @@ jobs:
         python-version: 3.6
     - name: Install dependencies
       run: |
+        apt-get install -y fftw fftw-dev
         python -m pip install --upgrade pip
         pip install tox tox-gh-actions
     - name: Run Tox Check