Support for force_py_version = "auto"

This will call `python` to look up its major version number.

doc/jedi-vim.txt

@@ -462,7 +462,7 @@ or set directly using this function, which has the same name as the variable:
 Function: `jedi#force_py_version(py_version)`
 
 Options: 2 or 3
-Default: 2
+Default: "auto" (will use sys.version_info from "python" in your $PATH)
 
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 7. Testing				*jedi-vim-testing*