Probably the easiest way to achieve this, though it's not 100% identical to Python's implementation, is to use the substr() function on the string. It differs to Python slicing because it takes the starting index and a length, rather than the starting and ending indices.
For your sapper example it would be:
text.substr(1,3) when using substr().
For completeness sake, to achieve the same effect as
print(stri[i:]) you can use the right() function, and for
print(stri[:i]) you can use the left() function.
All these functions are only available on the String class, GDScript doesn't have arrays that are as powerful as Python lists unfortunately.