Bump tomlkit from 0.5.5 to 0.5.7 in /docs

Bumps [tomlkit](https://github.com/sdispater/tomlkit) from 0.5.5 to 0.5.7.
- [Release notes](https://github.com/sdispater/tomlkit/releases)
- [Changelog](https://github.com/sdispater/tomlkit/blob/master/CHANGELOG.md)
- [Commits](python-poetry/tomlkit@0.5.5...0.5.7)

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

docs/requirements.txt

@@ -5,5 +5,5 @@ sphinx==2.2.0
 sphinx_autodoc_typehints==1.8.0
 recommonmark==0.6.0
 m2r==0.2.1
-tomlkit==0.5.5
+tomlkit==0.5.7
 docutils==0.14