Bump actions/setup-python from 2 to 3

Bumps [actions/setup-python](https://github.com/actions/setup-python) from 2 to 3.
- [Release notes](https://github.com/actions/setup-python/releases)
- [Commits](https://github.com/actions/setup-python/compare/v2...v3)

---
updated-dependencies:
- dependency-name: actions/setup-python
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/build.yml

@@ -21,7 +21,7 @@ jobs:
         uses: cisagov/setup-env-github-action@develop
       - uses: actions/checkout@v2
       - id: setup-python
-        uses: actions/setup-python@v2
+        uses: actions/setup-python@v3
         with:
           python-version: 3.9
       # We need the Go version and Go cache location for the actions/cache step,