Bump twisted from 17.5.0 to 19.7.0

Bumps [twisted](https://github.com/twisted/twisted) from 17.5.0 to 19.7.0.
- [Release notes](https://github.com/twisted/twisted/releases)
- [Changelog](https://github.com/twisted/twisted/blob/trunk/NEWS.rst)
- [Commits](twisted/twisted@twisted-17.5.0...twisted-19.7.0)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -78,7 +78,7 @@ six==1.10.0
 smmap2==2.0.5
 svn==0.3.46
 tablib==0.12.1
-Twisted==17.5.0
+Twisted==19.7.0
 txaio==2.10.0
 unicodecsv==0.14.1
 urllib3==1.23