There is a built-in ftoptions plugin which overrides some options depending on the file type, e.g. forces tabstospaces=true for Python and tabstospaces=false for Go.

The easiest option for you is just to disable this plugin:

set ftoptions false

Thanks for your answer that helps a lot.

Is there a way to configure this plugin ? this could be interesting.

But I can't understand why this plugin to this for python by default because it is well known that python need tabs.

It's a very simple plugin with hardcoded option settings: https://github.com/zyedidia/micro/blob/master/runtime/plugins/ftoptions/ftoptions.lua

It is compiled into micro, however you can override it without recompiling by copying the runtime/plugins/ftoptions directory from micro source to your ~/.config/micro/plug and modifying ftoptions.lua.
Well... I've been thinking that ~/.config/micro/plug/ftoptions would override the compiled-in plugin, but I see it doesn't: looks like both copies of the plugin are loaded, with no guarantee that ~/.config/micro/plug/ftoptions will take precedence. It seems worth fixing.
Anyway, you can name it e.g. ~/.config/micro/plug/ftoptions2 and set ftoptions false to ensure that only your ftoptions2 takes effect.

But I can't understand why this plugin to this for python by default because it is well known that python need tabs.

I'm not at all familiar with Python but a brief googling suggests that the recommended indent style for Python is 4 spaces.

Indeed sorry I was confused. Actually it's heavily debated.

This being said you gave me a good solution that allows me to continue using Micro, thanks for that !

Have a nice day