Unshallow git clone on RtD.

This is required for get_version to find the last tag.

Author: aaugustin

.readthedocs.yml

@@ -4,6 +4,9 @@ build:
   os: ubuntu-20.04
   tools:
     python: "3.10"
+  jobs:
+    post_checkout:
+      - git fetch --unshallow
 
 sphinx:
   configuration: docs/conf.py