Use ast.parse(type_comments=True) instead of file_tokens on Python 3.8+

[collinanderson]

Python 3.8 added the ability to include type comments in the abstract syntax tree.

Currently pyflakes uses tokenize to find the comments, which is slow. Using type_comments from ast when available would be much faster.

[asottile]

maybe a better option would be to drop the type comments support entirely -- now that python 2 is very obsolete

the problem with type_comments=True is pyflakes has no way to signal to flake8 that it needs a special ast parse

[collinanderson]

Ahh, yeah, that makes sense.

[asottile]

let's keep this open in case we decide to drop comments