Bump sphinx-autodoc-typehints from 1.8.0 to 1.9.0 in /docs

Bumps [sphinx-autodoc-typehints](https://github.com/agronholm/sphinx-autodoc-typehints) from 1.8.0 to 1.9.0.
- [Release notes](https://github.com/agronholm/sphinx-autodoc-typehints/releases)
- [Changelog](https://github.com/agronholm/sphinx-autodoc-typehints/blob/master/CHANGELOG.rst)
- [Commits](agronholm/sphinx-autodoc-typehints@1.8.0...1.9.0)

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

docs/requirements.txt

@@ -2,7 +2,7 @@
 # to generate documentation.
 
 sphinx==2.2.0
-sphinx_autodoc_typehints==1.8.0
+sphinx_autodoc_typehints==1.9.0
 recommonmark==0.6.0
 m2r==0.2.1
 tomlkit==0.5.8