Use Python 3 in `scripts/lint.py` (#5617)

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## Changes

- Use Python 3 in `scripts/lint.py`.

## Why

Some modern OS and distributions do not provide Python 2 by default.
This includes recent macOS versions.
I see that `scripts/generate_changelog.py` already uses Python 3, so I
don't think this change should be controversial.
Without this change, I am unable to run `./scripts/check.sh` on macOS
15.2 without installing Python 2 or adding alias for `python`
executable.

scripts/lint.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 """
 Runs custom linting on Rust code.
 """