LGTM

> Alternative: Add a new sys.implementation.debug_build flag.

According to the sys.implementation documentation and PEP 421, we can only add a private attribute without writing a PEP. But I find sys.implementation._debug_build too long and ``from sys import implementation; implementation._debug_build``(or ``from sys import implementation as i; i._debug_build``) is also not easy to write. So I'm +1 to sys.debug_build.

I left two trivial review comments on Rietveld.