3.13 asyncio DatagramTransport buffer size accounting regression.

[jsbronder]

Bug report

Bug description:

Commit 73e8637 added 8 to the buffer_size when
send could not be called right away. However, it did not complete this
accounting by removing 8 from the buffer size when sending did finally
complete.

I have a proposed fix and will file a PR shortly.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-135444: fix DatagramTransport buffer_size accounting #135445