fix subprocesspow keyboard interrupt not being catched

src/onionrproofs/subprocesspow.py

@@ -116,8 +116,11 @@ class SubprocessPOW:
 
         while True:
             # Break if shutdown received
+            try:
                 if pipe.poll() and pipe.recv() == 'shutdown':
                     break
+            except KeyboardInterrupt:
+                break
             # Load nonce into block metadata
             metadata['pow'] = nonce
             # Serialize metadata, combine with block data