py3: account for demand import difference between Python versions Our lazy importer for Python 3 will validate that modules are loadable before returning a stub module object. This is different from Python 2, which will always return a stub module object. While we could change behavior of the Python 3 demand importer, that seems like a problem for another day. This commit teaches test-extension.t about that difference in behavior. Differential Revision: https://phab.mercurial-scm.org/D5798