ci: no news fragment check for test-only PRs

.github/workflows/changelog.yml

@@ -1,10 +1,16 @@
 name: Changelog
 
 # Separate from tests.yml so that adding or removing the skip-changelog
-# label re-runs only this check, not the whole test matrix.
+# label re-runs only this check, not the whole test matrix.  A pull request
+# touching only tests and contributor docs changes nothing a user sees.
+# Such a pull request gets no check run at all, so do not make this check
+# required in branch protection: it would wait forever for one.
 on:
   pull_request:
     types: [opened, synchronize, reopened, labeled, unlabeled]
+    paths-ignore:
+      - "tests/**"
+      - "CONTRIBUTING.md"
 
 concurrency:
   group: changelog-${{ github.ref }}

CONTRIBUTING.md

@@ -18,7 +18,8 @@ named `<issue or PR number>.<type>.rst`, for example `42.bugfix.rst`. The
 types are listed under `[tool.towncrier]` in `pyproject.toml`: `breaking`,
 `removal`, `feature`, `bugfix`, `documentation`, `deps`, `internal`, `chore`
 and `security`. `towncrier create` makes the file for you. A pull request
-with nothing worth mentioning can be labelled `skip-changelog`.
+touching nothing but `tests/` or this file is not checked for one, and any
+other with nothing worth mentioning can be labelled `skip-changelog`.
 
 ## Development setup