Add python_[linter]_auto_pipenv options for python linters (fixes #1656)

When set to true, and the buffer is currently inside a pipenv,
GetExecutable will return "pipenv", which will trigger the existing
functionality to append the correct pipenv arguments to run each linter.

Defaults to false.

I was going to implement ale#python#PipenvPresent by invoking
`pipenv --venv` or `pipenv --where`, but it seemed to be abominably
slow, even to the point where the test suite wasn't even finishing
("Tried to run tests 3 times"). The diff is:

diff --git a/autoload/ale/python.vim b/autoload/ale/python.vim
index 7baae079..8c100d41 100644
--- a/autoload/ale/python.vim
+++ b/autoload/ale/python.vim
@@ -106,5 +106,9 @@ endfunction

" Detects whether a pipenv environment is present.
function! ale#python#PipenvPresent(buffer) abort
-    return findfile('Pipfile.lock', expand('#' . a:buffer . ':p:h') . ';') isnot# ''
+    let l:cd_string = ale#path#BufferCdString(a:buffer)
+    let l:output = systemlist(l:cd_string . 'pipenv --where')[0]
+    " `pipenv --where` returns the path to the dir containing the Pipfile
+    " if in a pipenv, or some error text otherwise.
+    return strpart(l:output, 0, 18) !=# "No Pipfile present"
 endfunction

Using vim's `findfile` is much faster, behaves correctly in the majority
of situations, and also works reliably when the `pipenv` command doesn't
exist.

1 files changed, 7 insertions, 0 deletions

diff --git a/test/command_callback/test_pyflakes_command_callback.vader b/test/command_callback/test_pyflakes_command_callback.vader
index efc925fe..92f83820 100644
--- a/test/command_callback/test_pyflakes_command_callback.vader
+++ b/test/command_callback/test_pyflakes_command_callback.vader
@@ -37,3 +37,10 @@ Execute(Setting executable to 'pipenv' appends 'run pyflakes'):
 
   AssertLinter 'path/to/pipenv',
   \ ale#Escape('path/to/pipenv') . ' run pyflakes %t',
+
+Execute(Pipenv is detected when python_pyflakes_auto_pipenv is set):
+  let g:ale_python_pyflakes_auto_pipenv = 1
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
+
+  AssertLinter 'pipenv',
+  \ ale#Escape('pipenv') . ' run pyflakes %t'