Bug: Tangling python code results in mixed tabs and spaces, incomaptible with python3 [9.1.4 (9.1.4-dist @ /home/ehere/emacs-scripts/org-9.1.4/lisp/)]

[Edmund Christian Herenz <christian.herenz@astro.su.se>]

The following python code uses only whitespaces for the different
indentdation levels:

a_list = ['elem1',
          'elem2',
          'elem3']

for elem in a_list:
    print(elem)
    for char in elem:
        print char

I enter this code into a SRC block with

#+BEGIN_SRC python :tangle blank_test.py

#+END_SRC

by pressing C-' inside the block (which opens the editing buffer
python-mode).  Then I press C-' again, after which I tangle the code
to blank_test.py by pressing C-u C-c C-v C-t.  The resulting file
blank_test.py will contain a mix of tabs and spaces for the different
intendation levels.  (I checked this with whitespace.el).

Above behaviour is a bug, since Python3 forbids mixing of spaces and
tabs. (Python2 is more relaxed about mixing of tabs and spaces). Thus,
the above code, syntactical correctly entered into an OrgSrc buffer,
will result in code that can not be run in python3 when tangled from
an org-mode file.

Emacs  : GNU Emacs 24.4.1 (x86_64-pc-linux-gnu, GTK+ Version 3.14.5)
 of 2017-09-12 on hullmann, modified by Debian
Package: Org mode version 9.1.4 (9.1.4-dist @ /home/ehere/emacs-scripts/org-9.1.4/lisp/)