ci(github-actions): add back 3.7 runner to pythonpackage

Author: Lee-W

.github/workflows/pythonpackage.yml

@@ -6,7 +6,7 @@ jobs:
   python-check:
     strategy:
       matrix:
-        python-version: [3.6, 3.8, 3.9]
+        python-version: [3.6, 3.7, 3.8, 3.9]
         platform: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.platform }}
     steps: