Update black to 19.10b0 (#6304)

changelog.d/6304.misc

contrib/experiments/test_messaging.py

mypy.ini

synapse/federation/sender/per_destination_queue.py

synapse/handlers/account_data.py

synapse/handlers/appservice.py

synapse/handlers/e2e_keys.py

synapse/handlers/federation.py

synapse/handlers/initial_sync.py

synapse/handlers/message.py

synapse/handlers/pagination.py

synapse/handlers/register.py

synapse/handlers/room.py

synapse/handlers/room_member.py

synapse/handlers/search.py

synapse/handlers/stats.py

synapse/handlers/sync.py

synapse/logging/_structured.py

synapse/push/bulk_push_rule_evaluator.py

synapse/push/emailpusher.py