Four free python resources

[martijnsublime]

Added four additional free resources. Three courses focused on Python for data science, one course focused on python programming.

Thank you for the resource.

[clarecorthell]

Thanks for adding a PR! The curriculum is an exclusive list of single best resources on each topic. Please let me know if you believe these are better than the existing resources, and why. Thanks!

[martijnsublime]

No problem. I think these resources are better/more appropriate for the following reasons:

(i) All the resources I added are free. lowering the barrier to entry. This is not true for all of the resources mentioned on the current list.
(ii) The resources I added focus on a combination of short online videos and interactive in-browser coding exercises. This is of course subjective, but I think this leads to a more engaging learning experience, certainly since on top of that the learning experience is supplemented with elements of gamification.
(iii) Continuous improvement: The added courses are taken by 100 thousands of students to date, and all providers have build-in feedback systems. This makes sure these courses are well tested with very few errors.

Hope this helps. Let me know in case you have any further questions.