Add mypy checks

.github/workflows/codestyle.yml

@@ -3,7 +3,7 @@ name: codestyle
 on: [push]
 
 jobs:
-  lint:
+  flake8:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
@@ -14,9 +14,39 @@ jobs:
           python-version: 3.9
 
       - name: install dependencies
-        run: |
-            make venv
+        run: make venv
 
-      - name: run codestyle
-        run: |
-            make lint
+      - name: run flake8
+        run: venv/bin/flake8 --max-line-length 120 dff/
+
+  black:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: set up python 3.9
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.9
+
+      - name: install dependencies
+        run: make venv
+
+      - name: run black
+        run: venv/bin/black --line-length=120 --check dff/
+
+  mypy:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: set up python 3.9
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.9
+
+      - name: install dependencies
+        run: make venv
+
+      - name: run mypy
+        run: venv/bin/mypy dff/