PR template: Add PR template to micropython-lib.

.github/pull_request_template.md

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+<!-- Thanks for submitting a Pull Request! We appreciate you spending the
+     time to improve MicroPython. Please provide enough information so that
+     others can review your Pull Request.
+
+     Before submitting, please read:
+     https://github.com/micropython/micropython/wiki/ContributorGuidelines
+
+     Please check any CI failures that appear after your Pull Request is opened.
+-->
+
+### Summary
+
+<!-- Explain the reason for making this change. What problem does the pull request
+     solve, or what improvement does it add? Add links if relevant,
+     especially links to open issues. -->
+
+### Testing
+
+<!-- Explain what testing you did, and on which boards/ports. If there are
+     boards or ports that you couldn't test, please mention this here as well.
+
+     If you leave this section empty then your Pull Request may be closed. -->
+
+### Trade-offs and Alternatives
+
+<!-- If the Pull Request has some negative impact (i.e. increased code size)
+     then please explain why you think the trade-off improvement is worth it.
+     If you can think of alternative ways to do this, please explain that here too.
+
+     Delete this heading if not relevant (i.e. small fixes) -->
+
+
+### Generative AI
+
+<!-- Please delete the answer below which doesn't apply, or write your own
+     answer with more details. -->
+
+I did not use generative AI tools when creating this PR.
+
+I used generative AI tools when creating this PR, but a human has checked the
+code and is responsible for the code and the description above.
+
+<!-- We require everyone to answer this question. If you delete this section or
+     ignore it then your PR may be closed.
+
+     For more information, consult the MicroPython Generative AI Policy:
+     https://github.com/micropython/micropython/wiki/ContributorGuidelines#generative-ai-policy
+     -->