Bump twisted from 18.9.0 to 19.7.0 in /docs

Bumps [twisted](https://github.com/twisted/twisted) from 18.9.0 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-18.9.0...twisted-19.7.0)

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

Author: dependabot[bot]

docs/requirements.txt

@@ -203,7 +203,7 @@ tornado==5.0.2
 tqdm==4.28.1
 traitlets==4.3.2
 twine==1.12.1
-Twisted==18.9.0
+Twisted==19.7.0
 urllib3==1.25.3
 verboselogs==1.7
 virtualenv==16.0.0