Bump tornado from 6.0.3 to 6.0.4

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.0.3 to 6.0.4.
- [Release notes](https://github.com/tornadoweb/tornado/releases)
- [Changelog](https://github.com/tornadoweb/tornado/blob/master/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.0.3...v6.0.4)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -8,7 +8,7 @@ pytest==4.6.6; python_version<"3.5"
 pytest==5.3.5; python_version>="3.5"
 six==1.14.0
 tornado==5.1.1; python_version<"3.6"
-tornado==6.0.3; python_version>="3.6"
+tornado==6.0.4; python_version>="3.6"
 trio==0.13.0; python_version>="3.6"
 outcome==1.0.1; python_version>="3.5"
 attrs==19.3.0; python_version>="3.5"