Forward TRAVIS_REPO_SLUG env var for GH publish notes

Fix #5749

Author: nicoddemus

tox.ini

@@ -118,7 +118,7 @@ commands = python scripts/release.py {posargs}
 description = create GitHub release after deployment
 basepython = python3.6
 usedevelop = True
-passenv = GH_RELEASE_NOTES_TOKEN TRAVIS_TAG
+passenv = GH_RELEASE_NOTES_TOKEN TRAVIS_TAG TRAVIS_REPO_SLUG
 deps =
     github3.py
     pypandoc