Remove dependency on astor

[kasium]

flake8.-simplify cannot be used with python 3.14 as astor is not compatible (see berkerpeksag/astor#233). However, as I see it, astor is no longer needed as ast.unparse exists (see berkerpeksag/astor#204).

@MartinThoma what do you think about giving ast.unparse a try and to add a python require of >=3.9?