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hackport-0.6.4: cabal/.github/PULL_REQUEST_TEMPLATE.md

---
Please include the following checklist in your PR:

* [ ] Patches conform to the [coding conventions](https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#conventions).
* [ ] Any changes that could be relevant to users have been recorded in the changelog.
* [ ] The documentation has been updated, if necessary.
* [ ] If the change is docs-only, `[ci skip]` is used to avoid triggering the build bots.

Please also shortly describe how you tested your change. Bonus points for added tests!