[pyupgrade] Add fix safety section to UP010 (#18838)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

Part of #15584

This adds a `Fix safety` section to [unnecessary-future-import
(UP010)](https://docs.astral.sh/ruff/rules/unnecessary-future-import/#unnecessary-future-import-up010)

The unsafety is determined here:

d9266284df/crates/ruff_linter/src/rules/pyupgrade/rules/unnecessary_future_import.rs (L128-L132)

Unsafe code example:
[playground](https://play.ruff.rs/c07d8c41-9ab8-4b86-805b-8cf482d450d9)
```py
from __future__ import (print_function,# ...
__annotations__)  # ...
```

Edit: It looks like there was already a PR for this, #17490, but I
missed it since they said `UP029` instead of `UP010` :/

## Test Plan

<!-- How was it tested? -->

N/A, no tests/functionality affected

crates/ruff_linter/src/rules/pyupgrade/rules/unnecessary_future_import.rs

@@ -30,6 +30,9 @@ use crate::{AlwaysFixableViolation, Applicability, Fix};
 /// print("Hello, world!")
 /// ```
 ///
+/// ## Fix safety
+/// This fix is marked unsafe if applying it would delete a comment.
+///
 /// ## Options
 /// - `target-version`
 ///