python: really start REPL also in emacs 25.1

[bmag]

Make python-start-or-switch-repl start a REPL when necessery also in Emacs 25.1. Fixes #6628.

The new version is supposed to work in Emacs 24.4, 24.5 and 25.1. For any daily python users: please confirm that this change doesn't break SPC m ' if you're on 24.4 or 24.5, or confirm that it makes SPC m ' work if you're on 25.1.

Edit: this PR also fixes #2872

[elvis-sik]

I am on 25.1

It works. I can open a REPL and send code to it.
However I am getting the following message when I open a REPL:

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

This appears when I open a .py file and again when I press SPC m '. The REPL doesn't open. Then I press SPC m ' again, no warning occurs, and the REPL opens.

[elvis-sik]

I see the code in this commit doesn't touch this variable, so this warning might not be related. It didn't happen up to a when the REPL stopped working, though (maybe it stopped because emacs was upgraded? Idk).

[lukeorland]

I got this in the Inferior Python buffer that opened up:

Unknown option: --
usage: python [option] ... [-c cmd | -m mod | file | -] [arg] ...
Try `python -h' for more information.

Process Python exited abnormally with code 2

[bmag]

@lukeorland probably not related to this PR, but there's not enough info to know for sure. If the same happens with SPC SPC run-python, then for sure the error has nothing to do with the PR.

[hujianxin]

We need this PR!!! 😄

[jecastro1]

I have tested this PR and it's working fine.

The warning is related to a bug in upstream emacs https://debbugs.gnu.org/cgi/bugreport.cgi?bug=24401. My current workaround is to set python-shell-completion-native-enable to nil, but it's not a real solution. However, I haven't tested what the guy in the bug report says.

[TheBB]

Thanks! Cherry-picked in develop. You can safely delete your branch.