python.el: Fix detection of native completion in Python 3 (bug #24401)

With Python 3.5, (python-shell-completion-native-get-completions ... "")
would return an empty list, causing python.el to think that native
completion was unavailable (the difference between Python 2 and Python 3
is due to https://bugs.python.org/issue25660).

* lisp/progmodes/python.el (python-shell-completion-native-try): Use "_"
  to check whether native completion is available instead of "".

1 files changed, 1 insertions, 1 deletions

diff --git a/lisp/progmodes/python.el b/lisp/progmodes/python.el
index 49f7bcf5df9..3fae3987b22 100644
--- a/lisp/progmodes/python.el
+++ b/lisp/progmodes/python.el
@@ -3296,7 +3296,7 @@ When a match is found, native completion is disabled."
          python-shell-completion-native-try-output-timeout))
     (python-shell-completion-native-get-completions
      (get-buffer-process (current-buffer))
-     nil "")))
+     nil "_")))
 
 (defun python-shell-completion-native-setup ()
   "Try to setup native completion, return non-nil on success."