Improve slice formatting (#5922)

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## Summary

- Remove space when start of slice is empty
- Treat unary op except `not` as simple expression

## Test Plan

Add some simple tests for unary op expressions in slice

Closes #5673

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/slice.py

@@ -70,12 +70,15 @@ def a():
 e00 = "e"[:]
 e01 = "e"[:1]
 e02 = "e"[: a()]
+e03 = "e"[:-1]
 e10 = "e"[1:]
 e11 = "e"[1:1]
 e12 = "e"[1 : a()]
+e13 = "e"[1:-1]
 e20 = "e"[a() :]
 e21 = "e"[a() : 1]
 e22 = "e"[a() : a()]
+e23 = "e"[a() : -1]
 e200 = "e"[a() :: ]
 e201 = "e"[a() :: 1]
 e202 = "e"[a() :: a()]