Github Actions: fix release automation

The PyPI publishing workflow was using ubuntu-18.04 which doesn't exist anymore.  Switched to use ubuntu-latest so it will work in the future.

For the 0.4.0 release, I published it manually.

.github/workflows/pypi.yml

@@ -5,7 +5,7 @@ on: release
 jobs:
   build-n-publish:
     name: Build and publish Python 🐍 distributions 📦 to PyPI
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
 
     steps:
     - uses: actions/checkout@master