On python<3.9, get rid of ast.Index before passing AST to PyiVisitor

[Akuli]

The version-specific special casing has ended up being repeated all over the place. This should clean it up.

While working on this, I noticed that there's a similar situation with ast.Num/ast.Str/etc and ast.Constant. For example, there's isinstance checks with ast.Num, which don't work on newer Pythons:

akuli@akuli-desktop:~$ python3.7 -c 'import ast; print(ast.dump(ast.parse("1")))'
Module(body=[Expr(value=Num(n=1))])
akuli@akuli-desktop:~$ python3.10 -c 'import ast; print(ast.dump(ast.parse("1")))'
Module(body=[Expr(value=Constant(value=1))], type_ignores=[])

I will fix these in a separate PR.

Closes #94