Bump tomlkit from 0.5.7 to 0.5.8 in /docs

Bumps [tomlkit](https://github.com/sdispater/tomlkit) from 0.5.7 to 0.5.8.
- [Release notes](https://github.com/sdispater/tomlkit/releases)
- [Changelog](https://github.com/sdispater/tomlkit/blob/master/CHANGELOG.md)
- [Commits](python-poetry/tomlkit@0.5.7...0.5.8)

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

docs/requirements.txt

@@ -6,7 +6,7 @@ sphinx_autodoc_typehints==1.8.0
 sphinx-click==2.3.0
 recommonmark==0.6.0
 m2r==0.2.1
-tomlkit==0.5.7
+tomlkit==0.5.8
 
 # Dependencies of our linter:
 typing-extensions==3.7.4