Remove parentheses around simple top-level expressions

[JelleZijlstra]

% cat parens.py 
(x)
(1)
(yield 42)
([])
({})
(a + b)
% black --unstable --diff parens.py
All done! ✨ 🍰 ✨
1 file would be left unchanged.

I think all of these parentheses should be removed.

We should keep parentheses around a top-level expression (ast.Expr) only if:

• There is a comment associated with the parentheses
• It's a ternary split into multiple lines
• Possibly other cases I haven't thought of