BUG: __getattr__ must raise AttributeError if name not found

PR matplotlib#20733 added module-level `__getattr__` functions to several
modules. All of the functions with the exception of the one in
`matplotlib.style.core` had a terminal `raise AttributeError` to
handle unmatched attributes.

The omission in `matplotlib.style.core` was probably
unintentional; it results in confusing and buggy behavior
such as:

```pycon
>>> import matplotlib.style.core
>>> if hasattr(matplotlib.style.core, '__warningregistry__'):
...     del matplotlib.style.core.__warningregistry__
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
AttributeError: __warningregistry__
```

This causes problems in the unit tests for astropy affiliated
packages. See astropy/astropy#12038.

Author: lpsinger

lib/matplotlib/style/core.py

@@ -33,6 +33,7 @@ def __getattr__(name):
     if name == "STYLE_FILE_PATTERN":
         _api.warn_deprecated("3.5", name=name)
         return re.compile(r'([\S]+).%s$' % STYLE_EXTENSION)
+    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")
 
 
 BASE_LIBRARY_PATH = os.path.join(mpl.get_data_path(), 'stylelib')