[qa] p2p segwit tests

mininode now supports witness transactions/blocks, blocktools
has a helper for adding witness commitments to blocks, and script
has a function to calculate hashes for signature under sigversion
1, used by segwit.

Py3 conversion by Marco Falke

Test to make sure upgraded nodes don't ask for non-wit blocks by
Gregory Sanders.

Author: sdaftuar

qa/pull-tester/rpc-tests.py

@@ -136,6 +136,7 @@
     'invalidtxrequest.py',
     'abandonconflict.py',
     'p2p-versionbits-warning.py',
+    'p2p-segwit.py',
     'segwit.py',
     'importprunedfunds.py',
     'signmessages.py',