Enable PIO Core tests on Python 3.8

.github/workflows/core.yml

@@ -8,7 +8,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        python-version: [2.7, 3.7]
+        python-version: [2.7, 3.7, 3.8]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2