Before:
  runtime ale_linters/python/pyflakes.vim

After:
  call ale#linter#Reset()

Execute(The pyflakes handler should handle basic errors):
  AssertEqual
  \ [
  \   {
  \     'lnum': 1,
  \     'col': 0,
  \     'text': 'undefined name ''foo''',
  \   },
  \   {
  \     'lnum': 1,
  \     'col': 7,
  \     'text': 'invalid syntax',
  \   },
  \ ],
  \ ale_linters#python#pyflakes#Handle(bufnr(''), [
  \   'test.py:1: undefined name ''foo''',
  \   'test.py:1:7: invalid syntax',
  \ ])