Fixes #5522 no typing module required on Focal

[kushaldas]

Status

Ready for review

Description of Changes

Fixes #5522

We need the typing module only on Xenial, but not on Focal.

Testing

• CI is green
• make build-debs-focal && molecule converge -s libvirt-staging-focal
• login to the app-staging and check if any systemctl service is failing

Deployment

Any special considerations for deployment? Consider both:

1. Upgrading existing production instances.
2. New installs.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

If you made changes to securedrop-admin:

• Linting and tests (make -C admin test) pass in the admin development container

If you made changes to the system configuration:

• Configuration tests pass

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

If you made changes to documentation:

• Doc linting (make docs-lint) passed locally

If you added or updated a code dependency:

Choose one of the following:

• I have performed a diff review and pasted the contents to the packaging wiki
• I would like someone else to do the diff review

[zenmonkeykstop]

Test plan passes, systemctl list-units --failed shows only vboxadd service failures (artifact of using converted virtualbox images). LGTM once CI passes (looks like staging-test-with-rebase flakes are not yet completely eliminated)

[emkll]

Inspected both xenial and focal packages produced by this branch, typing is included in the xenial virtualenv shipped by the built package, and not in the one shipped in the focal package.

Systemctl units are not failing in focal, and CI is green 🚀