ci: Fix imgtool CI workflow on main branch

[gustavonihei]

Summary

imgtool relies on packaging.version.LegacyVersion to parse image version numbers, and this has been removed in packaging 22.0.
This results in the following error in the Publish step:

+ python ../ci/compare_versions.py --old 1.9.0 --new 1.9.0
Traceback (most recent call last):
  File "/home/runner/work/mcuboot/mcuboot/scripts/../ci/compare_versions.py", line 15, in <module>
    from packaging.version import parse, LegacyVersion
ImportError: cannot import name 'LegacyVersion' from 'packaging.version' (/usr/local/lib/python3.10/dist-packages/packaging/version.py)

https://github.com/mcu-tools/mcuboot/actions/runs/3741361421/jobs/6350869462

Proposed fix based on pypa/pip-audit#427.

Impact

Should have no impact. Patch should bring CI back to normal.

Testing

CI build pass.

[utzig]

I think it'd be better to just get rid, of the LegacyVersion import and class check in compare_versions.py. An invalid version would just throw an exception, which is unlikely to happen since we control the versions.

[gustavonihei]

Closing in favor of solution proposed in #1564.