Introduce flake8 return

Based on !2281 (merged)

I've made one commit per rule. I'm not sure I agree with:

• RET502
• RET503.

Please tell me what you think, I can easily remove them.

ruff.toml

@@ -8,5 +8,7 @@ quote-style = "double"
 select = ["Q0", "F", "W", "E",
     # request-without-timeout
     "S113",
+    # flake8-return
+    "RET",
 ]
-ignore = ["E501"]
+ignore = ["E501", "RET502", "RET503"]