`_SelectorSocketTransport.writelines` is missing a flow control check allowing writes to fill memory until exhausted

[bdraco]

Bug report

Bug description:

This is the public issue for GHSA-fw89-6wjj-8j95

CPython versions tested on:

3.12

Operating systems tested on:

Linux, macOS

Linked PRs

• gh-127655: Ensure _SelectorSocketTransport.writelines pauses the protocol if needed #127656
• [3.13] gh-127655: Ensure _SelectorSocketTransport.writelines pauses the protocol if needed (GH-127656) #127663
• [3.12] gh-127655: Ensure _SelectorSocketTransport.writelines pauses the protocol if needed (GH-127656) #127664

[picnixz]

@bdraco I put all security labels but does it affect 3.9/3.10/3.11? (can't check for now)

[bdraco]

Its 3.12+ only since writelines was implemented in asyncio selector transport in 3.12