Update PR checklist with commented out code check (#18272)

Along with hopefully helping resolve these actual problems, a nudge for people to look at their own code might help catch other quick fixes before anyone else has to review a PR.

.github/PULL_REQUEST_TEMPLATE.md

@@ -13,6 +13,7 @@
 ## Checklist:
   - [ ] The code change is tested and works locally.
   - [ ] Local tests pass with `tox`. **Your PR cannot be merged unless tests pass**
+  - [ ] There is no commented out code in this PR.
 
 If user exposed functionality or configuration variables are added/changed:
   - [ ] Documentation added/updated in [home-assistant.io](https://github.com/home-assistant/home-assistant.io)