Pin Python dependencies to exact versions

[JanPetterMG]

This PR replaces compatible release specifiers ~= with exact version pins == for selected Python dependencies.

While loosening constraints was intended to allow safe patch updates, it introduces non-deterministic builds and can result in unexpected upstream changes being pulled into CI without explicit review. For infrastructure-related tooling and rendering libraries, even minor updates can introduce behavioral or output differences.

Key changes

• Pin the following packages to exact versions:
  • fonttools==4.61.1
  • matplotlib==3.10.8
  • pillow==12.1.1
  • tzdata==2025.3
  • httpx==0.28.1
• Replace ~= specifiers with == in requirements
• Ensure reproducible installs across CI and local environments

Impact

• Improves build reproducibility and determinism
• Prevents implicit adoption of newly released upstream versions
• Makes dependency updates an explicit and reviewable action
• Dependabot will still propose updates, but upgrades will now require intentional version bumps

No functional changes are introduced beyond dependency resolution behavior.