How important is the grade for a course I took, if I want to pursue graduate studies in that field?
Much of this will come down to what types of schools you want to apply to and what type of funding you hope to receive for graduate school. 

In the case of the graduate students we review at my school, GRE is an initial filter for classifying applicants as "acceptable" or "not acceptable." After that we do not look at GRE scores much. This means that a high GRE score gets you in the door and essentially nothing more. Letters of recommendation and grades in relevant classes are much more important to us. Your GRE scores are quite high. Unfortunately, graduate school success usually has little to do with how well you can perform on a single 4 hour test. Grades are a better reflection in my experience. (Especially subject-specific grades). 

I will be blunt: A C+ in a highly relevant course is not a good sign. There is no magic formula to overcome this. Admission committees will not really look highly upon a candidate who gets a C+ in a machine learning class because they could not learn Python. If you couldn't do it as an undergrad, what indicators are there that you will suddenly be blessed with the gift to learn Python and SK-Learn as a graduate student?

Understand that I'm not trying to be overly harsh on this. I am conveying the questions that an admissions committee must ask. 

I will add that sometimes we will admit an MS student who pays his/her own way. This is usually done in the case where we see a person who has potential, but not a lot to show for it. If their grade issues persist as a graduate student, it's not really a huge issue for us. They usually self-select to drop out of the program.