use python3 for workflows (#267)

Use python3 for running tests. Grist has supported python3 for a while,
and is slowly slowly starting to inch away from python2 support. Grist
now needs python3 when doing imports, although python2 is still supported
for regular document operation.

.github/workflows/docker_latest.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [2.7]
+        python-version: [3.9]
         node-version: [14.x]
     steps:
       - name: Check out the repo

.github/workflows/main.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [2.7]
+        python-version: [3.9]
         node-version: [14.x]
 
     steps: