Set up flake8 in CI

.github/workflows/build.yml

@@ -66,3 +66,39 @@ jobs:
         if: "matrix.python-version != '2.7' && matrix.python-version != 'pypy2'"
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+  lint:
+    name: ${{ matrix.toxenv }}
+    runs-on: ubuntu-latest
+
+    strategy:
+      matrix:
+        toxenv:
+          - flake8
+
+    steps:
+      - name: Git clone
+        uses: actions/checkout@v2
+
+      - name: Set up Python ${{ env.default_python || '3.9' }}
+        uses: actions/setup-python@v2
+        with:
+          python-version: "${{ env.default_python || '3.9' }}"
+
+      - name: Pip cache
+        uses: actions/cache@v2
+        with:
+          path: ~/.cache/pip
+          key: ${{ runner.os }}-pip-${{ matrix.toxenv }}-${{ hashFiles('tox.ini') }}
+          restore-keys: |
+            ${{ runner.os }}-pip-${{ matrix.toxenv }}-
+            ${{ runner.os }}-pip-
+
+      - name: Install dependencies
+        run: |
+          python -m pip install -U pip
+          python -m pip install -U setuptools wheel
+          python -m pip install -U tox
+
+      - name: Run ${{ matrix.toxenv }}
+        run: python -m tox -e ${{ matrix.toxenv }}