Bump sphinx from 3.5.3 to 3.5.4

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.5.3 to 3.5.4.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/4.x/CHANGES)
- [Commits](https://github.com/sphinx-doc/sphinx/commits/v3.5.4)

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

setup.py

@@ -7,7 +7,7 @@
         "flask-sqlalchemy",
         "marshmallow-sqlalchemy>=0.24.0",
     ],
-    "docs": ["marshmallow-sqlalchemy>=0.13.0", "Sphinx==3.5.3", "sphinx-issues==1.2.0"],
+    "docs": ["marshmallow-sqlalchemy>=0.13.0", "Sphinx==3.5.4", "sphinx-issues==1.2.0"],
     "lint": [
         "flake8==3.9.0",
         "flake8-bugbear==20.11.1",