Update publish-to-pypi.yml

Author: mattmanley

.github/workflows/publish-to-pypi.yml

@@ -9,7 +9,7 @@ on:
 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
       - name: Set up Python 3.6