Opting out of freethreading support

[davidhewitt]

In #4588 we are preparing to let users opt-in to full freethreading for their modules.

Later, in PyO3 0.24 or 0.25 we will make probably make that the default and have an opt-out.

I wonder, what kind of opt-out makes sense? Opting out of freethreading support for a module by not setting the module flag seems unhelpful, if the result is that users can still install that package on the freethreaded build and just get a degraded experience.

It seems to me like a better end result might be that the package in question refuses to install on the freethreaded build at all.

I guess we could do this by having supports_free_threaded = false expand to a compile error when building on the freethreaded build.

cc @ngoldbaum