ci: fix invalid format of Python version for PyPy, refactor

Author: samineer

.github/workflows/pythonpackage.yml

@@ -15,13 +15,20 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", "3.12"]
+        python-version:
+          - "3.6"
+          - "3.7"
+          - "3.8"
+          - "3.9"
+          - "3.10"
+          - "3.11"
+          - "3.12"
+          - "pypy3.6"
+          - "pypy3.7"
+          - "pypy3.8"
+          - "pypy3.9"
+          - "pypy3.10"
         os: [ubuntu-latest]
-        include:
-          - python-version: 3.6
-            os: ubuntu-20.04
-          - python-version: pypy3
-            os: ubuntu-20.04
 
 
     steps: