Remove unsupported versions of Python from packaging automation

Author: benfrancis

.github/workflows/pythonpackage.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.5, 3.6, 3.7]
+        python-version: [3.7]
     steps:
     - uses: actions/checkout@v1
     - name: Set up Python ${{ matrix.python-version }}