Add CI coverage for vmupdate formatting and pylint checks.

[nihalxkumar]

This extends the existing GitLab test job so vmupdate changes are checked with black and pylint after the unit tests run.

The patch also formats the current vmupdate files so the new black check starts from a clean baseline, and adds a focused pylint configuration for the checks that are actionable in this tree.

Mypy is intentionally not enabled in this PR because current upstream still has existing typing errors that need a separate cleanup before it can be a blocking CI check.

fixes: #213 (comment)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.74%. Comparing base (6786ad4) to head (aa2ca20).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #215      +/-   ##
==========================================
+ Coverage   71.72%   71.74%   +0.02%     
==========================================
  Files          12       12              
  Lines        1337     1338       +1     
==========================================
+ Hits          959      960       +1     
  Misses        378      378              

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[marmarek]

We have some template jobs for this already. Try:

include:
- file: /common.yml
  project: QubesOS/qubes-continuous-integration

...
lint:
  extends: .lint
  stage: checks
  variables:
    DIR: vmupdate

if pylint needs to be custom anyway, you can add SKIP_PYLINT: 1 here and keep pylint as you do now.