Bump sphinx from 3.0.3 to 3.0.4

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 3.0.3 to 3.0.4.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.0.3...v3.0.4)

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

setup.py

@@ -9,7 +9,7 @@
         'marshmallow-sqlalchemy>=0.13.0; python_version >= "3.6"',
         'marshmallow-sqlalchemy>=0.13.0,<0.19.0; python_version < "3.6"',
     ],
-    "docs": ["marshmallow-sqlalchemy>=0.13.0", "Sphinx==3.0.3", "sphinx-issues==1.2.0"],
+    "docs": ["marshmallow-sqlalchemy>=0.13.0", "Sphinx==3.0.4", "sphinx-issues==1.2.0"],
     "lint": [
         "flake8==3.8.2",
         'flake8-bugbear==20.1.4; python_version >= "3.5"',