Bump python version for CI tests to 3.9.

We run dias in python-3.8 on marimba, so presumably we should be
using that to test, but that's no longer available, so let's try 3.9.

.github/workflows/main.yml

@@ -15,10 +15,10 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
-    - name: Set up Python 3.8
+    - name: Set up Python 3.9
       uses: actions/setup-python@v1
       with:
-        python-version: 3.8
+        python-version: 3.9
 
     - name: Install black
       run: pip install black
@@ -40,10 +40,10 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
-    - name: Set up python 3.8
+    - name: Set up python 3.9
       uses: actions/setup-python@v1
       with:
-        python-version: 3.8
+        python-version: 3.9
 
     - name: Install apt dependencies
       run: |