DEV: Change to loadfile strategy for PyPy in CI (#3671)

It seems like the `loadfile` strategy to not use multiple workers for a test file seems to be more reliable than the tests on the current *main* branch. Let's hope that this finally brings us closer to CI not being unreliable with PyPy any more.

Author: stefan6419846

.github/workflows/github-ci.yaml

@@ -219,7 +219,7 @@ jobs:
       # Coverage on PyPy is skipped because running coverage with PyPy is slow and CPython test already provides
       # complete coverage data for the same code
       run: |
-        python -m pytest tests -n auto -vv -p no:benchmark -o faulthandler_timeout=60
+        python -m pytest tests -n auto -vv -p no:benchmark -o faulthandler_timeout=60 --dist=loadfile
       if: ${{ startsWith(matrix.python-version, 'pypy') }}
     - name: Rename coverage data file
       run: mv .coverage ".coverage.$RANDOM"