check-code: allow skipping hasattr check in py3-only code hasattr is safe in Python 3, and in an upcoming patch we can't use util.safehasattr.

profiling: allow loading profiling extension before everything else 6d642ecf1a89 makes profiler start early without loading extensions. That makes it impossible for an extension to add customized profilers. This patch adds a special case: if a profiler is not found but an extension with the same name could be loaded, load that extension first, and expect it to have a "profile" contextmanager method. This allows customized profilers and extension setup time is still profiled.

extensions: allow loading a whitelisted subset of extensions This feature will be used by the next patch.