Python support poetry (#3834)

* Add poetry support to python linters and black fixer.

* Update python.vim to detect poetry project.

* Update ale.vim, add an option for poetry `g:ale_python_auto_poetry`.

* Update ale-python.txt, add poetry support.

* Add and update poetry related tests.

Co-authored-by: unc0 <unc0@users.noreply.github.com>

test/fixers/test_black_fixer_callback.vader

@@ -55,3 +55,13 @@ Execute(Pipenv is detected when python_black_auto_pipenv is set):
   AssertEqual
   \ {'command': ale#Escape('pipenv') . ' run black -'},
   \ ale#fixers#black#Fix(bufnr(''))
+
+Execute(Poetry is detected when python_black_auto_poetry is set):
+  let g:ale_python_black_auto_poetry = 1
+  let g:ale_python_black_change_directory = 0
+
+  call ale#test#SetFilename('../test-files/python/poetry/whatever.py')
+
+  AssertEqual
+  \ {'command': ale#Escape('poetry') . ' run black -'},
+  \ ale#fixers#black#Fix(bufnr(''))