chore(deps-backend)(deps): update tinycss2 requirement from >=1.3.0 to >=1.5.1 in /backend

[dependabot]

Updates the requirements on tinycss2 to permit the latest version.

Release notes

Sourced from tinycss2's releases.

v1.5.1

• Include parsing tests in source tarball

Changelog

Sourced from tinycss2's changelog.

Version 1.5.1

Released on 2025-11-23.

• Include parsing tests in source tarball

Version 1.5.0

Released on 2025-11-19.

• Support most of CSS Color Level 5
• Fix tokenizer crash on escaped Dimension units and Function names

Version 1.4.0

Released on 2024-10-24.

• Support CSS Color Level 4

Version 1.3.0

Released on 2024-04-23.

• Support CSS nesting
• Deprecate parse_declaration_list, use parse_blocks_contents instead

Version 1.2.1

Released on 2022-10-18.

• Fix tests included in the source tarball

Version 1.2.0

Released on 2022-10-17.

• Drop support of Python 3.6
• Fix serialization of nested functions with no parameters
• Don’t use pytest plugins by default

... (truncated)

Commits

• f295a49 Version 1.5.1
• 282de7e Version 1.5.0
• a4a03f1 Add release action
• c59b394 Add documentation for color modules
• 36b3f6a Update supported and tested Python versions
• 5e6026f Merge pull request #64 from Kozea/css-color5
• 725d2b5 Simplify test_color_functions_5
• c3f7a92 Readd removed tests (oopsi)
• 1e47dd1 Manage None color in _build_color
• 4247cb7 Remove unused variables
• Additional commits viewable in compare view

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[overwrite00]

Closing in favor of a new PR targeting develop branch (following Git Flow). Dependabot will recreate this with the correct target.

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