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.

test/command_callback/test_flake8_command_callback.vader

@@ -1,5 +1,6 @@
 Before:
   call ale#assert#SetUpLinterTest('python', 'flake8')
+
   let b:bin_dir = has('win32') ? 'Scripts' : 'bin'
 
   WithChainResults ['3.0.0']
@@ -152,8 +153,16 @@ Execute(Using `python -m flake8` should be supported for running flake8):
 Execute(Setting executable to 'pipenv' should append 'run flake8'):
   let g:ale_python_flake8_executable = 'path/to/pipenv'
 
-  " FIXME: pipenv should check the vresion with flake8.
+  " FIXME: pipenv should check the version with flake8.
   WithChainResults []
   AssertLinter 'path/to/pipenv',
   \ ale#path#BufferCdString(bufnr(''))
   \   .  ale#Escape('path/to/pipenv') . ' run flake8 --format=default -'
+
+Execute(Pipenv is detected when python_flake8_auto_pipenv is set):
+  let g:ale_python_flake8_auto_pipenv = 1
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
+
+  AssertLinter 'pipenv',
+  \ ale#path#BufferCdString(bufnr(''))
+  \   . ale#Escape('pipenv') . ' run flake8 --format=default --stdin-display-name %s -'

test/command_callback/test_mypy_command_callback.vader

@@ -69,3 +69,11 @@ Execute(Setting executable to 'pipenv' appends 'run mypy'):
   \ ale#path#BufferCdString(bufnr(''))
   \   . ale#Escape('path/to/pipenv') . ' run mypy'
   \   . ' --show-column-numbers  --shadow-file %s %t %s'
+
+Execute(Pipenv is detected when python_mypy_auto_pipenv is set):
+  let g:ale_python_mypy_auto_pipenv = 1
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
+
+  AssertLinter 'pipenv',
+  \ ale#path#BufferCdString(bufnr(''))
+  \   . ale#Escape('pipenv') . ' run mypy --show-column-numbers  --shadow-file %s %t %s'

test/command_callback/test_prospector_command_callback.vader

@@ -10,3 +10,11 @@ Execute(Setting executable to 'pipenv' appends 'run prospector'):
   AssertLinter 'path/to/pipenv',
   \ ale#Escape('path/to/pipenv') . ' run prospector'
   \   .  '  --messages-only --absolute-paths --zero-exit --output-format json %s'
+
+Execute(Pipenv is detected when python_prospector_auto_pipenv is set):
+  let g:ale_python_prospector_auto_pipenv = 1
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
+
+  AssertLinter 'pipenv',
+  \ ale#Escape('pipenv') . ' run prospector'
+  \   .  '  --messages-only --absolute-paths --zero-exit --output-format json %s'

test/command_callback/test_pycodestyle_command_callback.vader

@@ -24,3 +24,10 @@ Execute(Setting executable to 'pipenv' appends 'run pycodestyle'):
 
   AssertLinter 'path/to/pipenv',
   \ ale#Escape('path/to/pipenv') . ' run pycodestyle  -'
+
+Execute(Pipenv is detected when python_pycodestyle_auto_pipenv is set):
+  let g:ale_python_pycodestyle_auto_pipenv = 1
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
+
+  AssertLinter 'pipenv',
+  \ ale#Escape('pipenv') . ' run pycodestyle  -'

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'

test/command_callback/test_pylint_command_callback.vader

@@ -68,3 +68,12 @@ Execute(Setting executable to 'pipenv' appends 'run pylint'):
   \ ale#path#BufferCdString(bufnr(''))
   \   . ale#Escape('path/to/pipenv') . ' run pylint'
   \   . '  --output-format text --msg-template="{path}:{line}:{column}: {msg_id} ({symbol}) {msg}" --reports n %s'
+
+Execute(Pipenv is detected when python_pylint_auto_pipenv is set):
+  let g:ale_python_pylint_auto_pipenv = 1
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
+
+  AssertLinter 'pipenv',
+  \ ale#path#BufferCdString(bufnr(''))
+  \   . ale#Escape('pipenv') . ' run pylint'
+  \   . '  --output-format text --msg-template="{path}:{line}:{column}: {msg_id} ({symbol}) {msg}" --reports n %s'

test/command_callback/test_pyls_command_callback.vader

@@ -38,3 +38,10 @@ Execute(Setting executable to 'pipenv' appends 'run pyls'):
   let g:ale_python_pyls_executable = 'path/to/pipenv'
 
   AssertLinter 'path/to/pipenv', ale#Escape('path/to/pipenv') . ' run pyls'
+
+Execute(Pipenv is detected when python_pyls_auto_pipenv is set):
+  let g:ale_python_pyls_auto_pipenv = 1
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
+
+  AssertLinter 'pipenv',
+  \ ale#Escape('pipenv') . ' run pyls'

test/command_callback/test_pyre_command_callback.vader

@@ -37,3 +37,10 @@ Execute(Setting executable to 'pipenv' appends 'run pyre'):
 
   AssertLinter 'path/to/pipenv',
   \ ale#Escape('path/to/pipenv') . ' run pyre persistent'
+
+Execute(Pipenv is detected when python_pyre_auto_pipenv is set):
+  let g:ale_python_pyre_auto_pipenv = 1
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
+
+  AssertLinter 'pipenv',
+  \ ale#Escape('pipenv') . ' run pyre persistent'

test/test_python_pipenv.vader

@@ -0,0 +1,13 @@
+Execute(ale#python#PipenvPresent is true when a pipenv environment is present):
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/pipenv/whatever.py')
+
+  AssertEqual
+  \  ale#python#PipenvPresent(bufnr('%')),
+  \  1
+
+Execute(ale#python#PipenvPresent is false true when no pipenv environment is present):
+  call ale#test#SetFilename('/testplugin/test/python_fixtures/no_pipenv/whatever.py')
+
+  AssertEqual
+  \  ale#python#PipenvPresent(bufnr('%')),
+  \  0