Update dependency radon to v4

This MR contains the following updates:

Package	Type	Update	Change
radon	dev-dependencies	major	^3.0 -> ^4.0

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pyproject.toml

@@ -62,7 +62,7 @@ sphinxcontrib-django = "^0.5.0"
 sphinx-autodoc-typehints = "^1.7"
 autopep8 = "^1.4"
 pylama = "^7.7"
-radon = "^3.0"
+radon = "^4.0"
 mypy = "^0.720.0"
 django-stubs = "^1.1"
 pytest = "^5.3"