test: remove type: comments in favour of actual annotations

Now that we require Python 3.6+, we should be using variable type
annotations directly rather than # type: comments.

Also takes care of the discarded value issue in p2p_message_capture.py.
See: https://github.com/bitcoin/bitcoin/pull/19509/files#r571674446.

test/functional/data/invalid_txs.py

@@ -57,7 +57,7 @@ class BadTxTemplate:
     __metaclass__ = abc.ABCMeta
 
     # The expected error code given by bitcoind upon submission of the tx.
-    reject_reason = ""  # type: Optional[str]
+    reject_reason: Optional[str] = ""
 
     # Only specified if it differs from mempool acceptance error.
     block_reject_reason = ""