Add flake8 as linter

.github/workflows/tests.yaml

@@ -1,21 +1,23 @@
-name: Run tests
-
-on: [push, pull_request]
-
-jobs:
-  build:
-
-    runs-on: ubuntu-latest
-
-    steps:
-    - uses: actions/checkout@v2
-    - name: Set up Python
-      uses: actions/setup-python@v2
-      with:
-        python-version: '3.x'
-    - name: Install dependencies
-      run: |
-        python -m pip install -U pip
-        python -m pip install -r dev-requirements.txt
-    - name: Test
-      run: make test
+name: Run tests
+
+on: [push, pull_request]
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up Python
+      uses: actions/setup-python@v2
+      with:
+        python-version: '3.x'
+    - name: Install dependencies
+      run: |
+        python -m pip install -U pip
+        python -m pip install -r dev-requirements.txt
+    - name: Linter
+      run: make lint
+    - name: Test
+      run: make test