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: https://github.com/rust-lang/rust/issues/135942

r? `@onur-ozkan`

src/conventions.md

@@ -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 -->
@@ -165,4 +186,5 @@ to the compiler.
   crate-related, often the spelling is changed to `krate`.
 
 [tcx]: ./ty.md
+
 [crates]: ./crates-io.md