Enable the RET503 lint in Ruff

.ruff.toml

@@ -222,7 +222,7 @@ select = [
     # flake8-return ('RET')
     "RET501",  # Do not explicitly `return None` in function if it is the only possible return value
     "RET502",  # Do not implicitly `return None` in function able to return non-`None` value
-#    "RET503",  # Missing explicit `return` at the end of function able to return non-`None` value
+    "RET503",  # Missing explicit `return` at the end of function able to return non-`None` value
 #    "RET504",  # Unnecessary assignment to `{name}` before `return` statement
 #    "RET505",  # Unnecessary `{branch}` after `return` statement
 #    "RET506",  # Unnecessary `{branch}` after `raise` statement

sphinx/domains/cpp/_parser.py

@@ -1909,7 +1909,7 @@ class DefinitionParser(BaseParser):
         self.skip_ws()
         if not self.skip_string('<'):
             self.fail("Expected '<' after 'template'")
-        while 1:
+        while True:
             pos = self.pos
             err = None
             try:

tests/test_builders/test_build_linkcheck.py

@@ -500,7 +500,7 @@ def custom_handler(
     def authenticated(
         method: Callable[[CustomHandler], None],
     ) -> Callable[[CustomHandler], None]:
-        def method_if_authenticated(self):
+        def method_if_authenticated(self: CustomHandler) -> None:
             if expected_token is None:
                 return method(self)
             elif not self.headers['Authorization']:
@@ -512,6 +512,7 @@ def custom_handler(
                 self.send_response(403, 'Forbidden')
                 self.send_header('Content-Length', '0')
                 self.end_headers()
+            return None
 
         return method_if_authenticated