Warn on 1-element tuple created without parens

[tkelman]

This confused a colleague of mine: a = b, creates a 1-element tuple in a. It's an easy typo to make if you're used to putting semicolons at the end of lines.

[tonyhffong]

by the time the ast reaches lint, these two statements are identical: a = (b,) a = b, and I have a suspicion that the first one may have a legitimate use. If Lint has access to the literal code text, there may be a chance to handle that.

[tkelman]

Yeah, the former is explicitly the syntax for a 1-element tuple, the latter gets implicitly lowered to the same thing but that could be pretty surprising.