setup: bump Sphinx version

Sphinx 3.0.4 doesn't version its dependencies correctly, and it fails with latest jinja 3.1.2:
  ImportError: cannot import name 'environmentfilter' from 'jinja2' (.../site-packages/jinja2/__init__.py)

Latest Sphinx version supports latest Jinja version and seems to render the
documentation correctly.

dev_requirements.txt

@@ -3,7 +3,7 b' pytest-sugar >= 0.9.2, < 0.10'
 pytest-benchmark >= 3.2.2, < 3.3
 pytest-localserver >= 0.5.0, < 0.6
 mock >= 3.0.0, < 4.1
-Sphinx >= 1.8.0, < 3.1
+Sphinx >= 1.8.0, < 5.2
 WebTest >= 2.0.6, < 2.1
 isort == 5.1.2
 pyflakes == 2.2.0