update twisted version

Change-Id: I9a573a648bc40eb61d2a497369fec072fec9803d
Reviewed-on: http://review.couchbase.org/c/perfrunner/+/138334
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Bo-Chun Wang <bo-chun.wang@couchbase.com>

Author: bochun

requirements.txt

@@ -31,7 +31,7 @@ setuptools==49.0.0
 spooky==2.0.0
 sqlalchemy==1.1.7
 sshtunnel==0.1.3
-twisted==16.2.0
+twisted==19.7.0
 validators==0.12.0
 xmltodict==0.11.0
 pyYAML==3.12