add one more item to pr

Author: rmorshea

.github/pull_request_template.md

@@ -9,3 +9,4 @@ Please update this checklist as you complete each item:
 - [ ] Tests have been included for all bug fixes or new functionality.
 - [ ] The `changelog.rst` has been updated with any significant changes.
 - [ ] GitHub Issues which may be closed by this Pull Request have been linked.
+- [ ] I have left irrelevant checklist items unchecked instead of removing them.