Validate DCoO PR signoff

Description

Automates check for PR authors having agreed to the last bullet point in the PR template. Copy of https://github.com/PythonistaGuild/.github/pull/1

[x] If code changes were made then they have been tested.
- [ ] I have updated the documentation to reflect the changes.
- [ ] I have updated the changelog with a quick recap of my changes.
[ ] This PR fixes an issue.
[x] This PR adds something new (e.g. new method or parameters).
[ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
[ ] This PR is not a code change (e.g. documentation, README, ...)
[x] I have read and agree to the Developer Certificate of Origin for this contribution