TST/MAINT: Further CI changes

This commit combines several changes to the CI:

    1. Removes the `max-parallel` argument
    2. Removes verbose output for pytest
    3. Add support for PyPy testing

Author: Kai-Striega

.github/workflows/pythonpackage.yml

@@ -7,10 +7,9 @@ jobs:
 
     runs-on: ${{ matrix.os }}
     strategy:
-      max-parallel: 4
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11"]
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11", 'pypy2.7', 'pypy3.7', 'pypy3.8', 'pypy3.9']
 
     steps:
     - uses: actions/checkout@v3
@@ -35,4 +34,4 @@ jobs:
     - name: Test with pytest
       run: |
         pip install pytest
-        pytest -v
+        pytest