bug#22897: 25.1.50; run-python warning with python3

[Mark Oteiza]

Hi,

When doing M-x run-python from emacs -Q, I get a warning. The contents
of *Warnings*:

  Warning (python): Your ‘python-shell-interpreter’ doesn’t seem to
  support readline, yet ‘python-shell-completion-native’ was t and
  "python" is not part of the
  ‘python-shell-completion-native-disabled-interpreters’ list.  Native
  completions have been disabled locally.

The contents of *Python*:

  Python 3.5.1 (default, Dec  7 2015, 12:58:09)
  [GCC 5.2.0] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  >>> python.el: native completion setup loaded
  >>> 

It appears that (python-shell-completion-native-setup) succeeds up to
the last form, (python-shell-completion-netive-try), which returns nil.
Changing the empty string in the latter function to "imp" for example
yields a completion.

Further, while (py-sh-co-native-try) returns a load of completions when
python-shell-interpreter is "python2" and returns none when "python3",
hitting TAB in an empty prompt with either python just inserts a tab,
which seems odd.

Inserting "imp", hitting TAB, DELing "imp" and hitting TAB again appears
to do grab all completions with python2, but will show

  0__dummy_completion__
  1__dummy_completion__

in *Completions*

In GNU Emacs 25.1.50.1 (x86_64-unknown-linux-gnu)
 of 2016-02-23 built on pathos
Repository revision: 96e32bbb736ec6e0a7278ae864098c7c812b05a4
Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --without-gconf --with-modules --with-x-toolkit=no
 'CFLAGS=-march=x86-64 -mtune=sandybridge -O2 -pipe
 -fstack-protector-strong --param=ssp-buffer-size=4'
 CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB X11 MODULES

Important settings:
  value of $LC_COLLATE: C
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix