Namespace packages _bootstrap.ModuleSpec.loader attributes are no longer None (#10376)

Namespace packages _bootstrap.ModuleSpec.loader attributes are no longer `None` _after_ calling the importlib._bootstrap._init_module_attrs function.

See:
* https://stackoverflow.com/questions/52869541/namespace-package-spec-loader-and-loader-attributes-not-set-to-none
* https://bugs.python.org/issue35181