test on 3.6 also

.github/workflows/ci.yaml

@@ -11,17 +11,23 @@ on:
 jobs:
   test:
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ['3.6', '3.x']
     steps:
     - uses: actions/checkout@v4
     - uses: actions/setup-python@v4
       with:
-        python-version: '3.x'
+        python-version: ${{ matrix.python-version }}
         cache: pip
     - name: install
       run: pip3 install --progress-bar off ruff coveralls
     - name: unittest
       run: coverage run --source pigwig --omit 'pigwig/tests/*' -m unittest -v
     - name: coveralls
       uses: coverallsapp/github-action@v2
+      if: ${{ matrix.python-version == '3.x' }}
+      with:
+        parallel-finished: true
     - name: ruff
       run: ruff check --output-format=github pigwig blogwig