COMP: Bump CI Ubuntu version to 20.04 for newer Python

Author: thewtex

.github/workflows/build-test-publish.yml

@@ -11,9 +11,9 @@ jobs:
     strategy:
       max-parallel: 3
       matrix:
-        os: [ubuntu-18.04, windows-2019, macos-10.15]
+        os: [ubuntu-20.04, windows-2019, macos-10.15]
         include:
-          - os: ubuntu-18.04
+          - os: ubuntu-20.04
             c-compiler: "gcc"
             cxx-compiler: "g++"
             cmake-build-type: "MinSizeRel"