DeprecationWarning for a couple of versions before removing public methods

[bitplane]

Hi

Just tracking down build errors and found the new version released yesterday! It broke starlette's test client.

I know it's my fault for not freezing deps, but any chance of a DeprecationWarning in future?

encode/starlette#2262

[agronholm]

The offending function had already been moved way back in v3.0.0 along with all the other thread-related functions, but curiously it was left out of the changelog – a human error. I have often added such warnings (most recently in create_memory_object_stream(), but this one slipped through the cracks. That said, anyone casually upgrading a SemVer project to the next major version without testing is just asking for trouble.