Lints updater code for flake8 F523

[conorsch]

Status

Ready for review

Description of Changes

F523 is use of str.format without the required a properly structured fmt
string. See https://flake8.pycqa.org/en/latest/user/error-codes.html

Testing

1. First, reproduce the issue: make flake8 on clean master fails
2. Then confirm make flake8 on this branch passes

In my testing, technically 2 failed due to the presence of old source dirs for prior versions of the RPM package, which contained older source code. Still, this'll satisfy CI.

Checklist

If you have made code changes

• Linter (make flake8) passes in the development environment (this box may
  be left unchecked, as flake8 also runs in CI)

If you have made changes to the provisioning logic

• All tests (make test) pass in dom0 of a Qubes install

• This PR adds/removes files, and includes required updates to the packaging
  logic in MANIFEST.in and rpm-build/SPECS/securedrop-workstation-dom0-config.spec

[emkll]

lgtm:

• Reproduced the issue: make flake8 on clean master fails
• Confirmed make flake8 on this branch passes