Python .tool versions (#3820)

* Also check for asdf-vm's .tool_versions file

A minimal python project may only be specifying a python version using a version management tool like asdf-vm, without providing other common python project configuration files. asdf-vm creates a single .tool_versions file in the managed directory. By checking for .tool_versions in addition to other common python config files we ensure that python linters (whose behaviour typically depends on a particular python version) will run with the same version of python used by the project. This will also be the same python version used by vim itself when it is run from inside the project's directories.

* add .tool_versions to ale-python-root documentation

This reflects the corresponding change to autoload/ale/python.vim

autoload/ale/python.vim

@@ -34,6 +34,7 @@ function! ale#python#FindProjectRootIni(buffer) abort
         \|| filereadable(l:path . '/Pipfile.lock')
         \|| filereadable(l:path . '/poetry.lock')
         \|| filereadable(l:path . '/pyproject.toml')
+        \|| filereadable(l:path . '/.tool_versions')
             return l:path
         endif
     endfor

doc/ale-python.txt

@@ -38,6 +38,7 @@ ALE will look for configuration files with the following filenames. >
   Pipfile.lock
   poetry.lock
   pyproject.toml
+  .tool_versions
 <
 
 The first directory containing any of the files named above will be used.