Update main.yaml (#561)

Use actions/checkout for better future proofing
Use new setup-python
Use python 3.8 as specified in pipfile

.github/workflows/main.yaml

@@ -4,11 +4,11 @@ jobs:
   build:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v1
+      - uses: actions/checkout@v2.3.4
       - name: Set up Python 3.7
-        uses: actions/setup-python@v1
+        uses: actions/setup-python@v2
         with:
-          python-version: 3.7
+          python-version: 3.8
       - name: Install Black
         run: pip install black
       - name: Run black --check .