Try adding python3-venv to an Aptfile

The goal is to restore the Digital Ocean (DO) pipeline while keeping
the existing `lint` command that now includes `sqlfluff` calls. The
assumption is that an Ubuntu Jammy image is being built and that when
an Aptfile is present, DO will use it before or during a build.

See https://docs.digitalocean.com/products/app-platform/reference/buildpacks/aptfile/

Hat tip to Dan Schultz for finding that documentation.

Issue #1410

.do/Aptfile

@@ -0,0 +1 @@
+python3-venv