Remove coverage tests from gh workflow.

As github workflows do not support Python2.7, we cannot run coverage
tests (they succeed only, if all Python3 and Python2 parts were tested)
nor Python2.7 tests on github.

We therefore remove both, tests for Python2.7 and coverage tests, from
the github workflow.

The default behavior of the local `tox.ini` still runs these tests and
therefore requires an installed Python2.7 version.

.github/workflows/tests.yml

 # This workflow will install Python dependencies, run tests and lint with different versions of Python
 # For more information see: https://help.github.com/actions/language-and-framework-guides/using-python-with-github-actions
 
+# As github support for Python2.7 has gone to meet its maker, we cannot
+# run successful coverage tests here.
+
 name: tests
 
 on:
@@ -19,28 +22,19 @@ jobs:
       matrix:
         os:
           - ubuntu-latest
-        config:
-          #- [ "2.7", "py27"]
-          #- [ "3.5", "py35"]
-          #- [ "3.6", "py36"]
-          #- [ "3.7", "py37"]
-          #- [ "3.8", "py38"]
-          - [ "3.9", "lint,clean,py27,py39,report"]
-          #- [ "pypy-2.7", "pypy"]
-          #- [ "pypy-3.7", "pypy3"]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
-      - name: Set up Python ${{ matrix.config[0] }}
+      - name: Set up Python (3.9)
         uses: actions/setup-python@v5
         with:
+          # We can give multiple python versions, but no py2.x any more
           python-version: |
-              "2.7"
-              ${{ matrix.config[0] }}
+            "3.9"
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
           pip install tox
       - name: Test with tox
         run: |
-          tox -e ${{ matrix.config[1] }}
+          tox run -e "lint,py39"