Bump GitPython dependency of "Deploy Website" workflow (versioned, MkDocs, Poetry) template

Since we are using GitPython ^3.1.20 when testing the versioning helper script used by this template, it seems best to specify the same for usage.

Author: per1234

workflow-templates/deploy-mkdocs-versioned-poetry.md

@@ -33,7 +33,7 @@ See the ["Deploy Website" workflow (MkDocs, Poetry) documentation](deploy-mkdocs
 
 1. Run this command:
    ```
-   poetry add --dev "gitpython@^3.1.19" "mike@^1.0.1"
+   poetry add --dev "gitpython@^3.1.20" "mike@^1.0.1"
    ```
 1. Commit the resulting `pyproject.toml` and `poetry.lock` files.