support for python 3.7

[ranahanocka]

currently, the installation fails for python 3.7 (but works just fine with python 3.6).

specifically, it complains:

cyminheap.c: In function ‘__Pyx_ExceptionSave’:
    cyminheap.c:2630:19: error: ‘PyThreadState’ has no member named ‘exc_type’
         *type = tstate->exc_type;