Add pull request template

Signed-off-by: Stephen Finucane <stephen@that.guru>

.github/pull_request_template.md

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+## Summary
+
+<!-- Summarise your change here -->
+
+## Tasks
+
+<!-- Mark tasks as complete or not applicable using [x] -->
+
+- [ ] Added unit tests
+- [ ] Added documentation for new features (where applicable)
+- [ ] Added release notes (using [`reno`](https://pypi.org/project/reno/))
+- [ ] Ran test suite and style checks and built documentation (`tox`)
+
+## Further details
+
+<!-- Note any further details here, where applicable -->