Add sphinx-theme-builder as valid Python build backend

See [here](https://sphinx-theme-builder.readthedocs.io/en/latest/filesystem-layout/#how-to-get-this-right) in the documentation, for example, for confirmation that sphinx-theme-builder is a Python build backend.

conda_smithy/linter/utils.py

@@ -67,6 +67,7 @@
     "pymsbuild",
     "meson-python",
     "scikit-build-core",
+    "sphinx-theme-builder",
     "maturin",
     "jupyter_packaging",
     "whey",