Add a pull request template (#2549)

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Co-authored-by: Daniel Szoke <szokeasaurusrex@users.noreply.github.com>

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Describe your PR here -->
+
+---
+
+## General Notes
+
+Thank you for contributing to `sentry-python`!
+
+Please add tests to validate your changes, and lint your code using `tox -e linters`.
+
+Running the test suite on your PR might require maintainer approval. Some tests (AWS Lambda) additionally require a maintainer to add a special label to run and will fail if the label is not present.
+
+#### For maintainers
+
+Sensitive test suites require maintainer review to ensure that tests do not compromise our secrets. This review must be repeated after any code revisions.
+
+Before running sensitive test suites, please carefully check the PR. Then, apply the `Trigger: tests using secrets` label. The label will be removed after any code changes to enforce our policy requiring maintainers to review all code revisions before running sensitive tests.