Singular.jl 0.21.0 released

Today Singular.jl 0.21.0 has been released.

Diff since v0.20.0

Merged pull requests:

• Update to AA 0.34, Nemo 0.38 (#734) (@fingolfin)
• Pin libsingular_julia_jll version (#736) (@fingolfin)
• Remove instance dispatches for *_type (#738) (@lgoettgens)
• Bump to 0.21 (#739) (@thofma)