Add the recent mypy fix to field.py to the template for this file

[miguelgrinberg]

This typing fix was incorrectly added to dsl/field.py. I missed that this file is generated, so its template should be fixed as well.

[github-actions]

The backport to 8.17 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-8.17 8.17
# Navigate to the new working tree
cd .worktrees/backport-8.17
# Create a new branch
git switch --create backport-2979-to-8.17
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 e616da96335f59849e193716132de8769acc9813
# Push it to GitHub
git push --set-upstream origin backport-2979-to-8.17
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-8.17

Then, create a pull request where the base branch is 8.17 and the compare/head branch is backport-2979-to-8.17.