Break before slice colon

**Summary** Break slices at the colon first, since the colon is separator with the lowest precedence and we're in a parenthesized context.

**Input**
```python
section_header_data = byte_array[byte_begin_index + byte_step_index * event_index : byte_begin_index + byte_step_index * (event_index + 1)]
```
**Black**
```python
section_header_data = byte_array[
    byte_begin_index
    + byte_step_index * event_index : byte_begin_index
    + byte_step_index * (event_index + 1)
]
```
**Current formatting**
```python
section_header_data = byte_array[
    byte_begin_index + byte_step_index * event_index : byte_begin_index
    + byte_step_index * (event_index + 1)
]
```
**Proposed formatting**
```python
section_header_data = byte_array[
    byte_begin_index + byte_step_index * event_index
    : byte_begin_index + byte_step_index * (event_index + 1)
]
```

This is another intentional black deviation, but i find it a clear style improvement.

This is consistent with adding a step:
```python
section_header_data2 = byte_array[
    byte_begin_index + byte_step_index * event_index
    : byte_begin_index + byte_step_index
    : section_size
]
```

As-is, this regresses trailing colon comments:

**in**
```python
c1 = "c"[
    1:  # e
    # f
    2
]
```
**out**
```python
c1 = "c"[
    1
    :  # e
    # f
    2
]
```

Fixes #7316

**Test Plan** Added the fixtures above.

crates/ruff_python_formatter/src/expression/expr_slice.rs

@@ -91,7 +91,7 @@ impl FormatNodeRule<ExprSlice> for FormatExprSlice {
         if !all_simple && lower.is_some() {
             space().fmt(f)?;
         }
-        token(":").fmt(f)?;
+        write!(f, [soft_line_break(), token(":")])?;
         // No upper node, no need for a space, e.g. `x[a() :]`
         if !all_simple && upper.is_some() {
             space().fmt(f)?;
@@ -125,7 +125,7 @@ impl FormatNodeRule<ExprSlice> for FormatExprSlice {
             if !all_simple && (upper.is_some() || step.is_none()) {
                 space().fmt(f)?;
             }
-            token(":").fmt(f)?;
+            write!(f, [soft_line_break(), token(":")])?;
             // No step node, no need for a space
             if !all_simple && step.is_some() {
                 space().fmt(f)?;