Fix artifact upload in pypi workflow

Since actions/upload-artifact version 4, Artifacts are immutable and we
can't upload multiple times to the same. Fixing this issue using:
https://github.com/actions/upload-artifact/blob/main/docs/MIGRATION.md

.github/workflows/pypi.yml

@@ -96,6 +96,7 @@ jobs:
 
       - uses: actions/upload-artifact@v4
         with:
+          name: artifact-${{ matrix.os }}
           path: ./wheelhouse/*.whl
 
   build_sdist:
@@ -114,8 +115,19 @@ jobs:
 
       - uses: actions/upload-artifact@v4
         with:
+          name: artifact-sdist
           path: dist/*.tar.gz
 
+  merge_artifacts:
+    runs-on: ubuntu-latest
+    needs: [build_wheels, build_sdist]
+    steps:
+      - name: Merge Artifacts
+        uses: actions/upload-artifact/merge@v4
+        with:
+          name: artifact
+          pattern: artifact-*
+
   upload_pypi:
     needs: [build_wheels, build_sdist]
     runs-on: ubuntu-latest