Detect Python proj. roots by Pipfile

"Pipfile" and "Pipfile.lock" files are also often located in Python module or
package directories and their presence is an okay heuristic for finding project
roots.

autoload/ale/python.vim

@@ -24,6 +24,8 @@ function! ale#python#FindProjectRootIni(buffer) abort
         \|| filereadable(l:path . '/mypy.ini')
         \|| filereadable(l:path . '/pycodestyle.cfg')
         \|| filereadable(l:path . '/flake8.cfg')
+        \|| filereadable(l:path . '/Pipfile')
+        \|| filereadable(l:path . '/Pipfile.lock')
             return l:path
         endif
     endfor