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/linter/test_pycodestyle.vader

@@ -31,3 +31,16 @@ Execute(Pipenv is detected when python_pycodestyle_auto_pipenv is set):
 
   AssertLinter 'pipenv',
   \ ale#Escape('pipenv') . ' run pycodestyle  -'
+
+Execute(Setting executable to 'poetry' appends 'run pycodestyle'):
+  let g:ale_python_pycodestyle_executable = 'path/to/poetry'
+
+  AssertLinter 'path/to/poetry',
+  \ ale#Escape('path/to/poetry') . ' run pycodestyle  -'
+
+Execute(Poetry is detected when python_pycodestyle_auto_poetry is set):
+  let g:ale_python_pycodestyle_auto_poetry = 1
+  call ale#test#SetFilename('../test-files/python/poetry/whatever.py')
+
+  AssertLinter 'poetry',
+  \ ale#Escape('poetry') . ' run pycodestyle  -'