Bump sphinx from 3.1.2 to 3.5.3

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.1.2 to 3.5.3.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](https://github.com/sphinx-doc/sphinx/commits)

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

Author: dependabot-preview[bot]

poetry.lock

@@ -40,7 +40,7 @@ responses = "^0.10.15"
 pytest-runner = "^5.2"
 towncrier = {version = "^19.2", optional = true}
 tox = "^3.19"
-sphinx = {version = "^3.1", optional = true}
+sphinx = {version = "^3.5", optional = true}
 sphinx_rtd_theme = {version = "^0.5.0", optional = true}
 pre-commit = {version = "^2.1", optional = true}