[gh actions] Update supported Python versions

Remove tests for Python < 3.7 due to updated rdflib version.

Author: mpsonntag

.github/workflows/run-tests.yml

@@ -35,7 +35,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-20.04]
-        python-version: ["3.6", "3.7"]
+        python-version: ["3.7"]
     steps:
       - uses: actions/checkout@v2
       - name: Setup Python ${{ matrix.python-version }}
@@ -61,7 +61,7 @@ jobs:
         # and fails on install. MacOS Python version 3.9 build will stay
         # disabled until this is resolved.
         #python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
-        python-version: ["3.6", "3.7", "3.8", "3.10"]
+        python-version: ["3.7", "3.8", "3.10"]
     steps:
       - uses: actions/checkout@v2
       - name: Setup Python ${{ matrix.python-version }}