Add missing type_comments attr to ast27.arguments

[Michael0x2a]

This pull request adds a missing attribute from the ast27.arguments class.

[gvanrossum]

Why does this not also need to be added to ast35?

[Michael0x2a]

After checking both the C definitions and testing typed_ast in the console, I found that...

• In ast35, type information is kept within:
  • ast35.FunctionDef.type_comments
  • ast35.AsyncFunctionDef.type_comments
  • ast35.arg.annotation
• In ast27, type information is kept within both:
  • ast27.FunctionDef.type_comments
  • ast27.arguments.type_comments (but only when using multi-line comments)

(I also found that ast27 doesn't have an arg node, like ast35 does -- in ast27, function arguments are typically represented by Name nodes).

Out of those five different fields, only ast27.arguments.type_comments is missing. My guess is that field is a relatively new addition, and we just forgot to update typeshed whenever we added support for multi-line comments.

[gvanrossum]

Cool!