chore(workflows): add PR checklist template and enforce checklist completion in pull requests

.github/pull_request_template.md

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+## PR Checklist
+
+Please confirm before requesting review:
+
+- [ ] I ran `pytest` locally without fails
+- [ ] I added/updated tests where needed
+- [ ] I updated documentation where needed
+
+Confirm before the final merge/rebase into master
+
+- [ ] Commits are sensibly squashed and rebased onto latest master
+- [ ] Standardtest run without difference (with recompiled cython code)

.github/workflows/require-checklist.yml

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+name: Require PR Checklist
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, reopened]
+
+jobs:
+  require-checklist:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: mheap/require-checklist-action@v2
+        with:
+          requireChecklist: true