Update ci.yaml (transferwise#169)

Author: jeet-parekh-wise

.github/workflows/ci.yaml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [ 3.6, 3.7, 3.8 ]
+        python-version: [ 3.6, 3.7, 3.8, 3.9 ]
 
     steps:
       - name: Checkout repository