bpo-29587: set exc.__context__ when calling gen.throw() (GH-19811)

Before this commit, if an exception was active inside a generator
when calling gen.throw(), then that exception was lost (i.e. there
was no implicit exception chaining).  This commit fixes that.

Author: cjerdonek

Misc/NEWS.d/next/Core and Builtins/2020-04-30-00-50-25.bpo-29587.oEwSq.rst

@@ -0,0 +1 @@
+Enable implicit exception chaining when calling :meth:`generator.throw`.

Objects/genobject.c

@@ -512,6 +512,12 @@ _gen_throw(PyGenObject *gen, int close_on_genexit,
     }
 
     PyErr_Restore(typ, val, tb);
+    if (gen->gi_exc_state.exc_type) {
+        Py_INCREF(gen->gi_exc_state.exc_type);
+        Py_XINCREF(gen->gi_exc_state.exc_value);
+        Py_XINCREF(gen->gi_exc_state.exc_traceback);
+        _PyErr_ChainExceptions(gen->gi_exc_state.exc_type, gen->gi_exc_state.exc_value, gen->gi_exc_state.exc_traceback);
+    }
     return gen_send_ex(gen, Py_None, 1, 0);
 
 failed_throw: