CI: stop towncrier from breaking releases

Author: mjpieters

.github/workflows/ci.yml

@@ -46,7 +46,7 @@ jobs:
         pip install -r requirements/towncrier.txt
     - name: Run docs spelling
       run: |
-        towncrier --yes
+        towncrier build --yes --version 99.99.99
         make doc-spelling
     - name: Prepare twine checker
       run: |