Make whole checklist a comment to avoid incomplete TODOs in PRs

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,7 +2,6 @@
 Thanks for submitting a PR, your contribution is really appreciated!
 
 Here is a quick checklist that should be present in PRs.
--->
 
 - [ ] Target the `master` branch for bug fixes, documentation updates and trivial changes.
 - [ ] Target the `features` branch for new features, improvements, and removals/deprecations.
@@ -13,3 +12,4 @@ Unless your change is trivial or a small documentation fix (e.g., a typo or rewo
 
 - [ ] Create a new changelog file in the `changelog` folder, with a name like `<ISSUE NUMBER>.<TYPE>.rst`. See [changelog/README.rst](https://github.com/pytest-dev/pytest/blob/master/changelog/README.rst) for details.
 - [ ] Add yourself to `AUTHORS` in alphabetical order.
+-->