update _makedirs in satpy.demo

[raybellwaves]

Given python 2 is dropped
https://satpy.readthedocs.io/en/latest/dev_guide/index.html#coding-guidelines

Could get rid of the comment/code about exist_ok=True and update the _makedirs function

https://github.com/pytroll/satpy/blob/master/satpy/demo/__init__.py#L59L68

[djhoese]

Similar to #1293, this could be a good "foot in the door" contribution. What do you think?

[djhoese]

@mraspaud I marked this as a bug since we don't really have a "cleanup" label. Maybe we need a cleanup label and a "component:tests" label?

[mraspaud]

Sure!