It IS built-in language syntax. It's defined in the PEP, that's built-in. It's syntax:
"Any Python script may have top-level comment blocks that MUST start with the line # /// TYPE where TYPE determines how to process the content. That is: a single #, followed by a single space, followed by three forward slashes, followed by a single space, followed by the type of metadata. Block MUST end with the line # ///. That is: a single #, followed by a single space, followed by three forward slashes. The TYPE MUST only consist of ASCII letters, numbers and hyphens."
"Any Python script may have top-level comment blocks that MUST start with the line # /// TYPE where TYPE determines how to process the content. That is: a single #, followed by a single space, followed by three forward slashes, followed by a single space, followed by the type of metadata. Block MUST end with the line # ///. That is: a single #, followed by a single space, followed by three forward slashes. The TYPE MUST only consist of ASCII letters, numbers and hyphens."
That's the syntax.
Built-in language syntax.