Fix ty typecheck in CI by using venv

Ty needs to find installed packages in a virtual environment.
Create a venv and activate it before running ty check.

Author: vinitkumar

.github/workflows/pythonpackage.yml

@@ -142,8 +142,11 @@ jobs:
 
     - name: Install dependencies
       run: |
-        uv pip install --system -e .
+        uv venv
+        uv pip install -e .
 
     - name: Run ty
-      run: uvx ty check json2xml tests
+      run: |
+        source .venv/bin/activate
+        uvx ty check json2xml tests