Yes, that's the rule black is implementing (assuming I'm reading your comment right and understanding its README right...). m[a + 1:b] is in danger of being read as "take the slice 1-to-b, add it to a, index m on that" - even though we know that isn't a well-typed set of operations, the spacing implies that's how it should be read, same as e.g. m[a + 1/b].

Spacings that don't conflict with precedence are m[a+1:b], m[a+1 : b], or m[a + 1 : b]. While the middle one makes precedence obvious, all three are acceptable. black picks the latter, and I think it picks that one because of another rule that it should prefer writing a + 1 instead of a+1.