compile fails with python 3.7.2

[ndbecker]

Compiling on python3.7.2 fails with lots of errors like:

    greenlet.c:495:11: error: ‘PyThreadState’ {aka ‘struct _ts’} has no member named ‘exc_traceback’; did you mean ‘curexc_traceback’?
       tstate->exc_traceback = target->exc_traceback;