Update to upcoming v0.3 release of SemialgebraicSets

[blegat]

• [x] https://github.com/JuliaHomotopyContinuation/PolynomialTestSystems.jl/pull/5
• [x] https://github.com/saschatimme/MixedSubdivisions.jl/pull/23

Closes https://github.com/JuliaHomotopyContinuation/HomotopyContinuation.jl/pull/553 Closes https://github.com/JuliaHomotopyContinuation/HomotopyContinuation.jl/pull/554 Closes https://github.com/JuliaHomotopyContinuation/HomotopyContinuation.jl/pull/555