Need python38 and python39

Building this as a library, which means it's going to be built for all
the current Python versions, and therefore needs those versions to be
present in the first place.

.github/workflows/ci.yml

@@ -5,4 +5,4 @@ jobs:
     uses: cygporter/workflows/.github/workflows/build-test.yml@v1
     with:
       cygport_file: python-rfc6555.cygport
-      bootstrap_packages: cygport python3
+      bootstrap_packages: cygport python38 python39