[MNT] Enhance PR template with detailed checklist items (#270)

Updated the pull request template to clarify self-review and testing
requirements.
The main motivation was that I prefer bots to self-identify. It's not
that hard to spot typically, and correct changes might normally still be
accepted, but it can help with prioritization or managing expectation on
response times.

---------

Co-authored-by: sourcery-ai[bot] <58596630+sourcery-ai[bot]@users.noreply.github.com>

Author: PGijsbers

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,10 +6,19 @@ Please include a summary of the change and include relevant motivation and conte
 
 _Please check all that apply. You can mark items as N/A if they don't apply to your change._
 
-- [ ] I have performed a self-review of my own code
+Always:
+- [ ] I have performed a self-review of my own pull request to ensure it contains all relevant information, and the proposed changes are minimal but sufficient to accomplish their task.
+
+Required for code changes:
+- [ ] Tests pass locally
 - [ ] I have commented my code in hard-to-understand areas, and provided or updated docstrings as needed
+- [ ] Changes are already covered under existing tests
+- [ ] I have added tests that cover the changes (only required if not already under coverage)
+
+If applicable:
 - [ ] I have made corresponding changes to the documentation pages (`/docs`)
-- [ ] I have added tests that cover the changes
-- [ ] Tests pass locally.
+
+Extra context:
+- [ ] This PR and the commits have been created autonomously by a bot/agent.
 
 nb. If tests pass locally but fail on CI, please try to investigate the cause. If you are unable to resolve the issue, please share your findings.