Before:
  runtime ale_linters/python/unimport.vim

After:
  call ale#linter#Reset()

Execute(The unimport handler should handle import warnings):
  AssertEqual
  \ [
  \   {
  \     'lnum': 9,
  \     'type': 'W',
  \     'text': 'unused: urllib.parse',
  \   },
  \ ],
  \ ale_linters#python#unimport#Handle(1, [
  \     'urllib.parse at path/to/file.py:9',
  \ ])