fetch entire repo to have tags for determining package version

By default the actions/checkout@v2 fetches only the last commit without
any other meta-data including tags. Thus when setuptools_scm attempts
to determine the version from git, it sees no tags, and cannot determine
the version or the distance from the last tag (for dev builds).

Author: wnienhaus

.github/workflows/publish.yml

@@ -13,6 +13,8 @@ jobs:
     steps:
     - name: Checkout
       uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
     - name: Set up Python
       uses: actions/setup-python@v2
       with: