Bump twisted from 16.0.0 to 19.2.1 in /doc

Bumps [twisted](https://github.com/twisted/twisted) from 16.0.0 to 19.2.1.
- [Release notes](https://github.com/twisted/twisted/releases)
- [Changelog](https://github.com/twisted/twisted/blob/twisted-19.2.1/NEWS.rst)
- [Commits](twisted/twisted@twisted-16.0.0...twisted-19.2.1)

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

doc/requirements.txt

@@ -2,7 +2,7 @@ sphinx
 sphinx_rtd_theme
 pbr==2.0.0
 oslo.config==2.1.0
-Twisted==16.0.0
+Twisted==19.2.1
 Flask==1.0
 Flask-HTTPAuth==2.5.0
 netaddr>=0.7.12