Pin version of black used in GHA.

Black rolls out style changes every year, and using "stable" means that
the check run on PRs might start formatting differently than we do
locally, or require a reformat of the codebase to make a PR submittable.

Pin to the version that we've been using. We should update to 23 at some
point, but we want to do that deliberately.

.github/workflows/lint.yml

@@ -11,6 +11,7 @@ jobs:
       - uses: actions/setup-python@v2
       - uses: psf/black@stable
         with:
+          version: ~=22.12
           src: "."
           options: "--check"