Added the default local virtualenv name for Pipenv.

Pipenv doesn't do local virtualenvs by default, it uses a special local
directory to store them all.

However, if you run Pipenv with the PIPENV_VENV_IN_PROJECT environment variable
set to 1, it creates the virtual environment in the root of the project, under
the name ".venv". This is why I've added this as a possible virtualenv dir
name.

autoload/ale/python.vim

@@ -6,6 +6,7 @@ let s:sep = has('win32') ? '\' : '/'
 let s:bin_dir = has('unix') ? 'bin' : 'Scripts'
 let g:ale_virtualenv_dir_names = get(g:, 'ale_virtualenv_dir_names', [
 \   '.env',
+\   '.venv',
 \   'env',
 \   've-py3',
 \   've',