Flake8-pyi

* support Python 3.9

* support flake8 3.8.0
* introduce Y091 (function body must not contain `raise`)
* introduce Y015 (attribute must not have a default value other than `...`)
* introduce Y092 (top-level attribute must not have a default value)

* update pyflakes dependency

* support Python 3.7
* add a check for non-ellipsis, non-typed arguments
* add checks for checking empty classes
* use --stdin-display-name as the filename when reading from stdin

* introduce Y011

* (release herp derp, don't use)