Don't include tests_isolated directory in cmd2 tarball published to PyPI

This is because those tests require the cmd2-ext-test plugin

MANIFEST.in

@@ -4,4 +4,5 @@ recursive-include tests *
 recursive-include docs *
 recursive-exclude docs/_build *
 recursive-exclude plugins *
+recursive-exclude tests_isolated *
 exclude .github .gitignore azure-pipelines.yml tasks.py