[#87] Update value for opacity800

[pylapp]

Note: Please transform - [ ] into - (NA) in the description when things are not applicable

Related issues

#87

Description

• @julien-deramond submtited a PR to update opacity800 (feat(opacity): update opacity800 from 0.88 to 0.80 #86)
• I checked his branch
• I merge into another branch so as to update the CHANGELOG and add a missing unit tests

Motivation & Context

See #87

Types of change

• Refactoring (non-breaking change)

Previews

Checklist

Contribution

• I have read the contributing guidelines

Accessibility

• (N/A) My change follows accessibility good practices

Design

• (N/A) My change respects the design guidelines of Orange Unified Design System

Development

• My change follows the developer guide
• I have added unit tests to cover my changes (optional)

Documentation

• My change introduces changes to the documentation and/or I have updated the documentation accordingly

Checklist (for Core Team only)

• The evolution have been tested and the project builds for iPhones and iPads
• Code review has been done by reviewers according to CODEOWNERS file
• (N/A) Design review has been done
• (N/A) Accessibiltiy review has been done
• (N/A) Q/A team has tested the evolution
• Documentation has been updated if relevant
• Internal files have been updated if relevant (like CONTRIBUTING, DEVELOP, THIRD_PARTY, CONTRIBUTORS, NOTICE)
• CHANGELOG has been updated respecting keep a changelog rules and reference the issues

[pylapp]

@ludovic35 It's ready for review :)