Use return_type not _return_type to fix an error on nightly

[mcabbott]

This change should be harmless on Julia 1.10 and 1.11, but fix an error on 1.12. It's an internal function so we did sign up to fix things as needed.

julia> VERSION
v"1.10.7"

julia> Core.Compiler.return_type(sin, Tuple{Int})
Float64

julia> Core.Compiler._return_type(sin, Tuple{Int})
Float64

julia> VERSION
v"1.11.2"

julia> Core.Compiler.return_type(sin, Tuple{Int})
Float64

julia> Core.Compiler._return_type(sin, Tuple{Int})
Float64

Master:

julia> VERSION
v"1.12.0-DEV.1731"

julia> Core.Compiler.return_type(sin, Tuple{Int})
Float64

julia> Core.Compiler._return_type(sin, Tuple{Int})
ERROR: MethodError: no method matching _return_type(::typeof(sin), ::Type{Tuple{Int64}})
You may have intended to import Base._return_type
The function `_return_type` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  _return_type(::Compiler.AbstractInterpreter, ::DataType)
   @ Base ../usr/share/julia/Compiler/src/typeinfer.jl:1244

[mcabbott]

Note aside that this package tests Zygote but does not constrain the version, hence CI is testing Zygote v0.7.0 here, with thunks from FluxML/Zygote.jl#966

[ToucheSir]

I don't recall why we opted to not constrain test dependencies in test/Project.toml, does anyone remember?