Ignore ruff F401 warnings for imports that are being used for existance checks

This is the obvious way to do this, try to import, use later if
available. Ruff doesn't understand that, so it complains. There just
isn't a better way to do this that doesn't end up being a complex dance
with `importlib.utils`, specs, etc.

setup.py

@@ -15,7 +15,7 @@ scm_args = {}
 HERE = os.path.dirname(__file__)
 if os.path.exists(os.path.join(HERE, '.git')):
     try:
-        import setuptools_scm
+        import setuptools_scm  # noqa: F401
     except ModuleNotFoundError:
         pass
     else: