Support categorical string backend

Author: aaronchucarroll

pandas/core/arrays/categorical.py

@@ -2744,11 +2744,15 @@ def _str_map(
         result = NumpyExtensionArray(categories.to_numpy())._str_map(f, na_value, dtype)
         return take_nd(result, codes, fill_value=na_value)
 
-    def _str_get_dummies(self, sep: str = "|"):
+    def _str_get_dummies(
+        self, sep: str = "|", dummy_na: bool = False, dtype: NpDtype = None
+    ):
         # sep may not be in categories. Just bail on this.
         from pandas.core.arrays import NumpyExtensionArray
 
-        return NumpyExtensionArray(self.astype(str))._str_get_dummies(sep)
+        return NumpyExtensionArray(self.astype(str))._str_get_dummies(
+            sep, dummy_na, dtype
+        )
 
     # ------------------------------------------------------------------------
     # GroupBy Methods