demandimport: import sub-module relatively as expected (issue5208) Before this patch, importing sub-module might (1) fail or (2) success but import incorrect module, because demandimport tries to import sub-module with level=-1 (on Python 2.x) or level=0 (on Python 3.x), which is default value of "level" argument at construction of "_demandmod" proxy object. (1) on Python 3.x, importing sub-module always fails to import existing sub-module (2) both on Python 2.x and 3.x, importing sub-module might import same name module on root level unintentionally On Python 2.x, existing sub-module is prior to this unexpected module. Therefore, this problem hasn't appeared. To import sub-module relatively as expected, this patch specifies "1" as import level explicitly at construction of "_demandmod" proxy object for sub-module.