Fix import of TerminateException in sim.py

This was broken in commit a5328f4, which got merged into master and
then released.

Author: rswarbrick

riscvmodel/sim.py

@@ -1,5 +1,4 @@
-from .model import Model, Memory
-from .isa import TerminateException
+from .model import Model, Memory, TerminateException
 
 import struct