Mypy: try to find folder containing mypy.ini to use as cwd. (#2385)

* When deciding which directory to run mypy from, prefer a folder with mypy.ini in it
* Add a test for mypy.ini-finding behaviour

ale_linters/python/mypy.vim

@@ -19,6 +19,15 @@ endfunction
 
 " The directory to change to before running mypy
 function! s:GetDir(buffer) abort
+    " If we find a directory with "mypy.ini" in it use that,
+    " else try and find the "python project" root, or failing
+    " that, run from the same folder as the current file
+    for l:path in ale#path#Upwards(expand('#' . a:buffer . ':p:h'))
+        if filereadable(l:path . '/mypy.ini')
+            return l:path
+        endif
+    endfor
+
     let l:project_root = ale#python#FindProjectRoot(a:buffer)
 
     return !empty(l:project_root)

test/command_callback/test_mypy_command_callback.vader

@@ -52,6 +52,15 @@ Execute(The mypy callbacks should detect virtualenv directories and switch to th
   \   . ' --show-column-numbers  '
   \   . '--shadow-file %s %t %s'
 
+Execute(The mypy callbacks should cd to directory containing mypy.ini if found):
+  silent execute 'file ' . fnameescape(g:dir . '/python_paths/with_mypy_ini_and_pytest_ini/tests/testsubfolder/my_tests.py')
+
+  AssertLinter 'mypy',
+  \   ale#path#CdString(ale#path#Simplify(g:dir . '/python_paths/with_mypy_ini_and_pytest_ini'))
+  \   . ale#Escape('mypy')
+  \   . ' --show-column-numbers  '
+  \   . '--shadow-file %s %t %s'
+
 Execute(You should able able to use the global mypy instead):
   silent execute 'file ' . fnameescape(g:dir . '/python_paths/with_virtualenv/subdir/foo/bar.py')
   let g:ale_python_mypy_use_global = 1