Bump tornado from 6.3.3 to 6.4.2 in /servant-quickcheck/doc (#1794)

Bumps [tornado](https://github.com/tornadoweb/tornado) from 6.3.3 to 6.4.2.
- [Changelog](https://github.com/tornadoweb/tornado/blob/v6.4.2/docs/releases.rst)
- [Commits](tornadoweb/tornado@v6.3.3...v6.4.2)

---
updated-dependencies:
- dependency-name: tornado
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

servant-quickcheck/doc/requirements.txt

@@ -20,6 +20,6 @@ snowballstemmer==1.2.1
 Sphinx==1.3.4
 sphinx-autobuild==0.5.2
 sphinx-rtd-theme==0.1.9
-tornado==6.3.3
+tornado==6.4.2
 watchdog==0.8.3
 wheel==0.38.1