include standard cimport for pythread.pxd in cpython cimport

Cython/Includes/cpython/__init__.pxd

@@ -143,6 +143,7 @@ from cpython.function cimport *
 from cpython.method cimport *
 from cpython.weakref cimport *
 from cpython.getargs cimport *
+from cpython.pythread cimport *
 
 # Python <= 2.x
 from cpython.cobject cimport *