Add --show-traceback to stubtest_third_party (#14511)

Author: Avasam

tests/stubtest_third_party.py

@@ -108,6 +108,7 @@ def run_stubtest(dist: Path, *, verbose: bool = False, ci_platforms_only: bool =
                 "mypy.stubtest",
                 "--mypy-config-file",
                 temp.name,
+                "--show-traceback",
                 # Use --custom-typeshed-dir in case we make linked changes to stdlib or _typeshed
                 "--custom-typeshed-dir",
                 str(dist.parent.parent),