Lack of mathematical work during PhD and coping up with the requirements of postdoc positions
If you have the knowledge, but not the "qualification" in those areas, make sure that your letters of recommendation say that. If you don't have the knowledge, it might be a bit easier to improve Python skills than math skills. 

Alternatively, you might be able to get someone to tutor  you in the math area and make sure your advisor and other recommenders know to mention it in the future. 

A third option is to focus your statement of purpose and other application materials on your knowledge of these things, rather than just listing courses.