REF: use another ctor for fastpath

Also use __init__ instead of __new__ for simplicity.

Author: TomAugspurger

pandas/core/dtypes/dtypes.py

@@ -152,18 +152,25 @@ class CategoricalDtype(ExtensionDtype):
     _metadata = ['categories', 'ordered']
     _cache = {}
 
-    def __new__(cls, categories=None, ordered=False, fastpath=False):
+    def __init__(self, categories=None, ordered=False):
+        self._finalize(categories, ordered, fastpath=False)
+
+    @classmethod
+    def _from_fastpath(cls, categories=None, ordered=False):
+        self = cls.__new__(cls)
+        self._finalize(categories, ordered, fastpath=True)
+        return self
+
+    def _finalize(self, categories, ordered, fastpath=False):
         from pandas.core.indexes.base import Index
 
         if categories is not None:
             categories = Index(categories, tupleize_cols=False)
             # validation
-            cls._validate_categories(categories, fastpath=fastpath)
-            cls._validate_ordered(ordered)
-        categorical = object.__new__(cls)
-        categorical._categories = categories
-        categorical._ordered = ordered
-        return categorical
+            self._validate_categories(categories)
+            self._validate_ordered(ordered)
+        self._categories = categories
+        self._ordered = ordered
 
     def __hash__(self):
         # _hash_categories returns a uint64, so use the negative