Use unshallow Git fetch for setuptools-scm (Read the Docs) (#491)

* Test unshallow.

* Add comment.

.readthedocs.yml

@@ -6,9 +6,10 @@ build:
     python: mambaforge-4.10
   jobs:
     post_checkout:
-      # reference: https://github.com/SciTools/cf-units/pull/272
-      # perform a "git fetch --unshallow" when the cf-units repo
-      # becomes incomplete i.e., there is a .git/shallow.
+      # The SciTools/cf-units repository is shallow i.e., has a .git/shallow,
+      # therefore complete the repository with a full history in order
+      # to allow setuptools-scm to correctly auto-discover the version.
+      - git fetch --unshallow
       - git fetch --all
     pre_install:
       # create a "common" link to the underlying rtd conda environment,