[Python] C++ Extension Types aren't correctly exposed in pyarrow

[sjperkins]

Describe the bug, including details regarding any error messages, version, and platform.

Version: master branch (11.0.0)
Platform: Ubuntu 20.04

Neither __arrow_ext_class__ nor __arrow_ext_scalar_class__ are exposed on BaseExtensionType.

This results in the following sort of errors when trying to access a C++ ExtensionArray/ExtensionType from pyarrow:

AttributeError: 'pyarrow.lib.BaseExtensionType' object has no attribute '__arrow_ext_class__'

See the following, for example:

• [Python] Running a substrait plan that includes an extension type returns results where the column.chunks attribute has an AttributeError #32291
• ARROW-638: [C++] Complex Number Support via ExtensionTypes #10565 (comment)

Component(s)

Python

[jorisvandenbossche]

Issue resolved by pull request 33802
#33802