Specify flake8

.github/workflows/pythonapp.yml

@@ -27,6 +27,7 @@ jobs:
       run: |
         mamba install -y nbmake pytest-xdist line_profiler  # add'l packages for notebook tests.
         mamba install --file requirements.txt --file requirements-extra.txt
+        mamba install flake8  # specify flake8 to avoid unknown error
         python -m pip install -e .
     - name: Lint with flake8 for pull requests
       if: github.event_name == 'pull_request'