fetch tags before towncrier docs build

.readthedocs.yaml

@@ -12,6 +12,7 @@ build:
       # in that case exit the command with 183 to skip the build
       - MSG=`git --no-pager log --pretty="tformat:%s -- %b" -1`; case $MSG in *"skip rtd"*|*"rtd skip"*) exit 183;; *);; esac
     post_install:
+      - git fetch --tags
       - towncrier build --keep
 
 sphinx: