pqc/.github/pull_request_template.md

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#### Manually checked properties
* [ ] Generated Github workflow (run ``.github/workflows/generate_workflows.py``) (new schemes)
* [ ] No stringification macros
* [ ] Output-parameter pointers in functions are on the left
* [ ] Negative return values on failure of API functions (within restrictions of FO transform).
* [ ] variable declarations at the beginning (except in `for (size_t i=...`)
* Optional:
  * [ ] All integer types are of fixed size, using `stdint.h` types (including `uint8_t` instead of `unsigned char`)
  * [ ] Integers used for indexing are of size `size_t`