Bump sphinxcontrib-serializinghtml from 1.1.5 to 1.1.9

Bumps [sphinxcontrib-serializinghtml](https://github.com/sphinx-doc/sphinxcontrib-serializinghtml) from 1.1.5 to 1.1.9.
- [Release notes](https://github.com/sphinx-doc/sphinxcontrib-serializinghtml/releases)
- [Changelog](https://github.com/sphinx-doc/sphinxcontrib-serializinghtml/blob/master/CHANGES)
- [Commits](sphinx-doc/sphinxcontrib-serializinghtml@1.1.5...1.1.9)

---
updated-dependencies:
- dependency-name: sphinxcontrib-serializinghtml
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

doc/requirements.txt

@@ -4,5 +4,5 @@ sphinxcontrib-devhelp==1.0.2; python_version >= "3.5"
 sphinxcontrib-htmlhelp==2.0.1; python_version >= "3.5"
 sphinxcontrib-jsmath==1.0.1; python_version >= "3.5"
 sphinxcontrib-qthelp==1.0.3; python_version >= "3.5"
-sphinxcontrib-serializinghtml==1.1.5; python_version >= "3.5"
+sphinxcontrib-serializinghtml==1.1.9; python_version >= "3.5"
 django-typer==0.1.0