Update the Pull_request_template.md file

[Mariatta]

Changed the template to be inside an <!-- --> tag.
Let contributors know that the PR title should include bpo number.

[brettcannon]

The other option is to include a checklist that people can check off to make sure they have everything covered. Not sure if that's better than the comment since people will still need to change the body for the final commit, but it might help lead to people forgetting stuff less.

[brettcannon]

"Please read this comment in its entirety. It's quite important."

[Mariatta]

Thanks for the quick review!
Can I make the checklist inside the comment tag?
So we don't actually need to 'tick' each box.
Eventually the bots will tell them everything they need to know.

[brettcannon]

I don't think there's a point to making checklist if people can't physically check stuff off, so don't worry about it.

[Mariatta]

Ok. Let's try out this new PR template for now, we can think about the checklist later.