Nautilus and Nemo don't recognize python 3 files

Nautilus and Nemo use two different icons for python files:

But I want them to show the python icon for all python files.

mimetype gives the same result for both files:

$ mimetype *.py
buy_test.py:         text/x-python
candlestick_test.py: text/x-python

file -i gives different outputs:

$ file -i buy_test.py 
buy_test.py: text/x-python; charset=us-ascii
$ file -i candlestick_test.py 
candlestick_test.py: text/x-objective-c; charset=us-ascii

The output for candlestick_test.py is wrong but this file is shown with the python icon. No idea why.

Somebody here had the same problem: Ubuntu file manager is not showing proper icons

But the answer he got, doesn't work for me:

$ grep -r 'text/x-python' /usr/share/thumbnailers

No output.

I think the main diffenrence between these python files is the shebang:

$ head -n2 buy_test.py 
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
$ head -n2 candlestick_test.py 
# -*- coding: utf-8 -*-
"""

When I use this shebang (python 2) ...

#!/usr/bin/env python

... there is a python icon, too.

How do Nautilus and Nemo choose file icons?

EDIT:

Some additional information:

There is a python icon if I use one of these shebangs:

#!/usr/bin/env python
#!/usr/bin/python

There is no python icon if I use one of these shebangs:

#!/usr/bin/env python3
#!/usr/bin/python3

So it looks like python3 is a problem.

EDIT 2:

xdg-mime query filetype FILENAME returns the MIME type matching the shebang:

$ xdg-mime query filetype buy_test.py 
text/x-python3

$ xdg-mime query filetype candlestick_test.py 
text/x-python

And when I change candlestick_test.py's file extension to .py3, the python icon disappears and xdg-mime query filetype returns text/x-python3.

EDIT 3:

I found a new source of information:

$ gio info ~/python/buy_test.py | grep icon
  standard::icon: text-x-python3, text-x-generic
  standard::symbolic-icon: text-x-python3-symbolic, text-x-generic-symbolic, text-x-python3, text-x-generic
$ gio info ~/python/candlestick_test.py | grep icon
  standard::icon: text-x-python, text-x-generic
  standard::symbolic-icon: text-x-python-symbolic, text-x-generic-symbolic, text-x-python, text-x-generic

So I guess, I have to link text-x-python3 files to text-x-python icons somehow.