Pythonic Substring Anagram Index
index is a singular noun that doesn't represent of group of things, therefor it isn't a good name for an array.

Your method is named anagram_indices and returns an array, I like to call the returned array results, so it's clear that the array are the anagram indices.

Using i and x is very confusing, so much that it made me question my own knowledge of the enumerate function. I expected i to be the index and x to be the letter, while it's the opposite. You should rename those with better variable names. Actually, using single letter variable names is almost never a good idea, there are some edge cases. I expect a variable named i to represent an index, for example.

Your algorithm itself is fine, but the naming is very confusing, which is kind of a problem.