Add pull request template

Author: lossyrob

.github/pull_request_template.md

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+**Related Issue(s):** #
+
+
+**Description:**
+
+
+**PR Checklist:**
+
+- [ ] Code is formatted (run `scripts/format`)
+- [ ] Tests pass (run `scripts/test`)
+- [ ] This PR maintains or improves overall codebase code coverage.
+- [ ] Changes are added to the [CHANGELOG](https://github.com/stac-utils/pystac/blob/develop/CHANGELOG.md). See [the docs](https://pystac.readthedocs.io/en/latest/contributing.html#changelog) for information about adding to the changelog.