added flake8 pip install statement

.github/workflows/python-package.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       max-parallel: 1
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: [3.6, 3.8]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}
@@ -18,6 +18,7 @@ jobs:
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip wheel setuptools
+        python -m pip install flake8
         if [ -f requirements.dev.txt ]; then pip install -r requirements.dev.txt;  else pip install .[testing]; fi
     - name: Lint with flake8
       run: |