Fix CI: remove Python 3.8 from test matrix

Remove Python 3.8 from GitHub Actions test matrix to match package requirement of Python >=3.9. This resolves the CI error "Package 'withoutbg' requires a different Python: 3.8.18 not in '>=3.9'".

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: kocabiyik

.github/workflows/ci.yml

@@ -12,15 +12,11 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        python-version: ['3.8', '3.9', '3.10', '3.11', '3.12']
+        python-version: ['3.9', '3.10', '3.11', '3.12']
         exclude:
           # Reduce matrix for faster CI
-          - os: windows-latest
-            python-version: '3.8'
           - os: windows-latest
             python-version: '3.9'
-          - os: macos-latest
-            python-version: '3.8'
           - os: macos-latest
             python-version: '3.9'