docs: Update CONTRIBUTING.md to note Python checks

CONTRIBUTING.md

@@ -19,15 +19,28 @@ frustration later on.
 All submissions, including submissions by project members, require review.
 
 ### Coding Style
-For consistency, please ensure that all files are formatted by `clang-format` using the [.clang-format](.clang-format) style file.  You can verify this by running:
+For consistency, please ensure that all TypeScript/JavaScript files
+are formatted by `clang-format` using the
+[.clang-format](.clang-format) style file.  You can verify this by
+running:
+
 ```shell
 npm run gulp check-format
 ```
-To reformat all source files, run:
+To reformat all TypeScript/JavaScript source files, run:
 ```shell
 npm run gulp format
 ```
 
+Python code is linted and formatted using
+[ruff](https://github.com/astral-sh/ruff) and typechecked using
+[mypy](https://mypy-lang.org/).  To verify, run:
+
+```shell
+pip install nox
+nox -s lint format mypy
+```
+
 ### The small print
 Contributions made by corporations are covered by a different agreement than
 the one above, the

pyproject.toml

@@ -28,6 +28,7 @@ exclude = [
     "^src/",
     "/guide_video_recorder/",
     "^docs/",
+    "^build/",
 ]
 
 [[tool.mypy.overrides]]