Merge pull request #2942 from rodrigc/twisted-17.9.0

[MRG+1] Bump Twisted requirement on Python 3 to 17.9.0 to catch many Python 3 fixes.

requirements-py3.txt

-Twisted >= 15.5.0
+Twisted >= 17.9.0
 lxml>=3.2.4
 pyOpenSSL>=0.13.1
 cssselect>=0.9