test: Fix “local variable 'e' is assigned to but never used”

flake8 F841 lints, as of flake8 3.6.0

test/functional/feature_assumevalid.py

@@ -72,7 +72,7 @@ class AssumeValidTest(BitcoinTestFramework):
                 break
             try:
                 p2p_conn.send_message(msg_block(self.blocks[i]))
-            except IOError as e:
+            except IOError:
                 assert not p2p_conn.is_connected
                 break