Update models/artifact.py to set OriginalChecksums attribute as Optional

[sthiriet]

Description

Mark OriginalChecksums attrs as Optional.

Fixes #210

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How has it been tested ?

Please describe the tests that you ran to verify your changes. Please also list any relevant details for your test configuration

• Run existing tests

Checklist:

• My PR is ready for prime time! Otherwise use the "Draft PR" feature
• All commits have a correct title
• Readme has been updated
• [] Quality tests are green (see Codacy)
• [] Automated tests are green (see pipeline)

[anancarv]

lgtm