demandimport: support "absolute_import" for external libraries (issue4029) Before this patch, demandimport of Mercurial may fail to load external libraries using "from __future__ import absolute_import": for example, importing "foo" in "bar.baz" module will load "bar.foo" if it exists, even though "absolute_import" is enabled in "bar.baz" module. So, extensions for Mercurial can't use such external libraries. This patch saves "level" of import request for on-demand module loading in the future: default value of level is -1, and level is 0 when "absolute_import" is enabled. "level" value is passed to built-in import function in "_demandmod._load()" and it should load target module correctly. This patch changes only one "_demandmod" construction case other than cases below: - construction in "_demandmod._load()" this code path should be used only in relative sub-module loading case - constructions other than patched one in"_demandimport()" these code paths shouldn't be used in "level != -1" case

hghave: add "py3k" feature to check whether test runs with Python 3.x This patch adds "py3k" feature to check whether test runs with Python 3.x. This check is needed for portability of test code: for example, in the default, modules are imported relatively first with python 2.x, but imported absolutely with Python 3.x.