manually unshallow the repository on RTD (#7961)

RTD is deprecating the feature flag we made use of before.

Author: keewis

.readthedocs.yaml

@@ -7,6 +7,7 @@ build:
   jobs:
     post_checkout:
       - (git --no-pager log --pretty="tformat:%s" -1 | grep -vqF "[skip-rtd]") || exit 183
+      - git fetch --unshallow || true
     pre_install:
       - git update-index --assume-unchanged doc/conf.py ci/requirements/doc.yml