Rollup merge of #135950 - Kobzol:tidy-python-improvements, r=onur-ozkan

Tidy Python improvements

Fixes display of Python formatting diffs in tidy, and refactors the code to make it simpler and more robust. Also documents Python formatting and linting in the Rustc dev guide.

Fixes: rust-lang/rust#135942

r? `@onur-ozkan`

Author: matthiaskrgr

src/conventions.md

@@ -1,4 +1,4 @@
-This file offers some tips on the coding conventions for rustc.  This
+This file offers some tips on the coding conventions for rustc. This
 chapter covers [formatting](#formatting), [coding for correctness](#cc),
 [using crates from crates.io](#cio), and some tips on
 [structuring your PR for easy review](#er).
@@ -25,6 +25,7 @@ pass the <!-- date-check: nov 2022 --> `--edition=2021` argument yourself when c
 `rustfmt` directly.
 
 [fmt]: https://github.com/rust-dev-tools/fmt-rfcs
+
 [`rustfmt`]:https://github.com/rust-lang/rustfmt
 
 ## Formatting C++ code
@@ -40,6 +41,26 @@ When modifying that code, use this command to format it:
 This uses a pinned version of `clang-format`, to avoid relying on the local
 environment.
 
+## Formatting and linting Python code
+
+The Rust repository contains quite a lof of Python code. We try to keep
+it both linted and formatted by the [ruff][ruff] tool.
+
+When modifying Python code, use this command to format it:
+```sh
+./x test tidy --extra-checks=py:fmt --bless
+```
+
+and the following command to run lints:
+```sh
+./x test tidy --extra-checks=py:lint
+```
+
+This uses a pinned version of `ruff`, to avoid relying on the local
+environment.
+
+[ruff]: https://github.com/astral-sh/ruff
+
 <a id="copyright"></a>
 
 <!-- REUSE-IgnoreStart -->
@@ -84,7 +105,7 @@ Using `_` in a match is convenient, but it means that when new
 variants are added to the enum, they may not get handled correctly.
 Ask yourself: if a new variant were added to this enum, what's the
 chance that it would want to use the `_` code, versus having some
-other treatment?  Unless the answer is "low", then prefer an
+other treatment? Unless the answer is "low", then prefer an
 exhaustive match. (The same advice applies to `if let` and `while
 let`, which are effectively tests for a single variant.)
 
@@ -124,7 +145,7 @@ See the [crates.io dependencies][crates] section.
 # How to structure your PR
 
 How you prepare the commits in your PR can make a big difference for the
-reviewer.  Here are some tips.
+reviewer. Here are some tips.
 
 **Isolate "pure refactorings" into their own commit.** For example, if
 you rename a method, then put that rename into its own commit, along
@@ -165,4 +186,5 @@ to the compiler.
   crate-related, often the spelling is changed to `krate`.
 
 [tcx]: ./ty.md
+
 [crates]: ./crates-io.md