fix: removal of pyarrow.lib.PyExtensionType in pyarrow 21 (#3581)

* only check for PyExtensionType if version is less than 21

* should be less than

* fix typo

* do a try, except block instead

* better comments

Author: ikrommyd

src/awkward/_connect/pyarrow/conversions.py

@@ -186,13 +186,16 @@ def popbuffers(paarray, awkwardarrow_type, storage_type, buffers, generate_bitma
     if awkwardarrow_type is not None:
         paarray = paarray.storage
     ### Beginning of the big if-elif-elif chain!
+    try:
+        if isinstance(storage_type, pyarrow.lib.PyExtensionType):
+            raise ValueError(
+                "Arrow arrays containing pickled Python objects can't be converted into Awkward Arrays"
+            )
+    except AttributeError:
+        # PyExtensionType is deprecated in pyarrow 21.0.0.
+        pass
 
-    if isinstance(storage_type, pyarrow.lib.PyExtensionType):
-        raise ValueError(
-            "Arrow arrays containing pickled Python objects can't be converted into Awkward Arrays"
-        )
-
-    elif isinstance(storage_type, pyarrow.lib.ExtensionType):
+    if isinstance(storage_type, pyarrow.lib.ExtensionType):
         # AwkwardArrowType should already be unwrapped; this must be some other ExtensionType.
         assert not isinstance(storage_type, AwkwardArrowType)
         # In that case, just ignore its logical type and use its storage type.
@@ -495,12 +498,16 @@ def popbuffers(paarray, awkwardarrow_type, storage_type, buffers, generate_bitma
 def form_popbuffers(awkwardarrow_type, storage_type):
     ### Beginning of the big if-elif-elif chain!
 
-    if isinstance(storage_type, pyarrow.lib.PyExtensionType):
-        raise ValueError(
-            "Arrow arrays containing pickled Python objects can't be converted into Awkward Arrays"
-        )
+    try:
+        if isinstance(storage_type, pyarrow.lib.PyExtensionType):
+            raise ValueError(
+                "Arrow arrays containing pickled Python objects can't be converted into Awkward Arrays"
+            )
+    except AttributeError:
+        # PyExtensionType is deprecated in pyarrow 21.0.0.
+        pass
 
-    elif isinstance(storage_type, pyarrow.lib.ExtensionType):
+    if isinstance(storage_type, pyarrow.lib.ExtensionType):
         # AwkwardArrowType should already be unwrapped; this must be some other ExtensionType.
         assert not isinstance(storage_type, AwkwardArrowType)
         # In that case, just ignore its logical type and use its storage type.