Update PR template (#13723)

The old one was long and weirdly incomplete. It had placeholder text
like "link to instructions".

Author: hauntsaninja

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,22 +1,12 @@
-### Have you read the [Contributing Guidelines](https://github.com/python/mypy/blob/master/CONTRIBUTING.md)?
-
-(Once you have, delete this section. If you leave it in, your PR may be closed without action.)
-
-### Description
-
-<!--
-If this pull request closes or fixes an issue, write Closes #NNN" or "Fixes #NNN" in that exact
-format.
--->
+<!-- If this pull request fixes an issue, add "Fixes #NNN" with the issue number. -->
 
 (Explain how this PR changes mypy.)
 
-## Test Plan
-
 <!--
-If this is a documentation change, rebuild the docs (link to instructions) and review the changed pages for markup errors.
-If this is a code change, include new tests (link to the testing docs). Be sure to run the tests locally and fix any errors before submitting the PR (more instructions).
-If this change cannot be tested by the CI, please explain how to verify it manually.
+Checklist:
+- Read the [Contributing Guidelines](https://github.com/python/mypy/blob/master/CONTRIBUTING.md)
+- Add tests for all changed behaviour.
+- If you can't add a test, please explain why and how you verified your changes work.
+- Make sure CI passes.
+- Please do not force push to the PR once it has been reviewed.
 -->
-
-(Write your test plan here. If you changed any code, please provide us with clear instructions on how you verified your changes work.)