BUG: __getattr__ must raise AttributeError if name not found (again)

PR matplotlib#20806 added another instance of a module-level `__getattr__`
function that was missing a final `raise AttributeError` to catch
unknown attributes. This is the same kind of bug as matplotlib#20822.

See also astropy/astropy#12038.

Author: lpsinger

lib/matplotlib/cm.py

@@ -34,6 +34,7 @@ def __getattr__(name):
         _api.warn_deprecated("3.5", name=name,
                              alternative="rcParams['image.lut']")
         return _LUTSIZE
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
 
 
 _LUTSIZE = mpl.rcParams['image.lut']