adapt pytest to using pluggy (current master)

--HG--
branch : pluggy1

pytest.py

@@ -12,7 +12,7 @@ if __name__ == '__main__': # if run as a script or by 'python -m pytest'
 # else we are imported
 
 from _pytest.config import main, UsageError, _preloadplugins, cmdline
-from _pytest.core import hookspec_opts, hookimpl_opts
+from pluggy import hookspec_opts, hookimpl_opts
 from _pytest import __version__
 
 _preloadplugins() # to populate pytest.* namespace so help(pytest) works