Pyright: create a more isolated environment in CI for testing stubs (#10121)

pyrightconfig.testcases.json

@@ -1,6 +1,7 @@
 {
     "$schema": "https://raw.githubusercontent.com/microsoft/pyright/main/packages/vscode-pyright/schemas/pyrightconfig.schema.json",
     "typeshedPath": ".",
+    "venv": ".venv",
     "include": [
         "test_cases",
         "stubs/**/@tests/test_cases"