Support Stackless releases after 2013

This commit adds code to use the new tracing options of
Stackless versions released after 2013. Debugging should work reliable now.

We are going to release Stackless 2.7.6 very soon now. Releases of Stackless 3.x and 2.8 will follow. These releases contain fixes and enhancements that help debugger integration. Especially it is no longer required to monkey patch Stackless.