ci: update pytest workflow to trigger on all pull requests, including forks (#141)

Author: aaronsteers

.github/workflows/pytest_matrix.yml

@@ -23,8 +23,8 @@ jobs:
     name: Pytest (All, Python ${{ matrix.python-version }}, ${{ matrix.os }})
     # Don't run on forks. Run on pushes to main, and on PRs that are not from forks.
     if: >
-      (github.event_name == 'push' && github.ref == 'refs/heads/main') ||
-      (github.event.pull_request.head.repo.fork == false)
+      github.event_name == 'pull_request' ||
+      (github.event_name == 'push' && github.ref == 'refs/heads/main')
     strategy:
       matrix:
         python-version: [