asyncio: expose WriteTransport.get_write_buffer_limits on all Python versions

[Akuli]

#5779 (comment) shows code that uses this method. It works on 3.6, 3.7 and 3.8, but doesn't type-check without this PR on those versions.

Fixes #5779

[JelleZijlstra]

https://docs.python.org/3/library/asyncio-protocol.html#asyncio.WriteTransport.get_write_buffer_limits says it was new in 3.4.2, not sure where we got 3.9 from.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[AlexWaygood]

https://docs.python.org/3/library/asyncio-protocol.html#asyncio.WriteTransport.get_write_buffer_limits says it was new in 3.4.2, not sure where we got 3.9 from.

It was a weird one where somebody noticed on BPO a few weeks ago that these had been documented forever, but not actually implemented in the code. So they were added the other week, and backported to 3.9.