It was pretty unwieldy and handled some disparate parts of the
exception-raising process.  I think things are a bit cleaner now:
- cxx_unwind.cpp handles C++ unwinding semantics
- unwinding.cpp converts C stacks to Python stacks
- exceptions.cpp takes Python stack frames and handles them appropriately

So for throwing a C++ exception, it starts out in cxx_unwind.cpp, which
then hands off the C frames to unwinding.cpp, which then hands off the
Python frames to exceptions.cpp.  When we get exceptions not via uncaught
C++ exceptions (ie explicitly handled C++ exceptions or CAPI exceptions),
those go directly into exceptions.cpp.  There are also non-exception cases
that we want to get the Python stack trace (ex sys._getframe), and those
are handled by unwinding.cpp