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.