GH-33801: [Python] Expose C++ ExtensionTypes/ExtensionArrays in pyarrow

[sjperkins]

• Closes[Python] C++ Extension Types aren't correctly exposed in pyarrow #33801

Rationale for this change

C++ Extension Types are not correctly exposed in pyarrow

What changes are included in this PR?

__arrow_ext_class__ and __arrow_ext_scalar_class__ have been moved from ExtensionType to BaseExtensionType in types.pxi.

Are these changes tested?

Yes, a test has been added to test_cython.py. There may be better locations for this, but the existing cython testing machinery here is useful for generating a C++ extension type on the fly.

Are there any user-facing changes?

I don't believe there are any user-facing changes as __arrow_ext_class__ and __arrow_ext_scalar_class__ are moved into a base class.

[github-actions]

• Closes: [Python] C++ Extension Types aren't correctly exposed in pyarrow #33801

[github-actions]

⚠️ GitHub issue #33801has been automatically assigned in GitHub to PR creator.

[jorisvandenbossche]

Thanks a lot for this PR! (and sorry for the slow feedback)

Looking good, added some comments.

[jorisvandenbossche]

@sjperkins FYI I also opened an issue about having custom subclasses for extension types implemented in C++ (instead of using the base class BaseExtensionType/ExtensionArray): #33997

[sjperkins]

@sjperkins FYI I also opened an issue about having custom subclasses for extension types implemented in C++ (instead of using the base class BaseExtensionType/ExtensionArray): #33997

Thanks @jorisvandenbossche for mentioning this, as well as the review. I'm intending to address your review comments next week.

[sjperkins]

Is there anything else needed from me on this PR?

[jorisvandenbossche]

No, nothing else needed, this looks good! (and thanks for the ping :))

[sjperkins]

Thanks for the review and merging @jorisvandenbossche

[ursabot]

Benchmark runs are scheduled for baseline = b40fb2c and contender = 1333545. 1333545 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.4% ⬆️0.0%] test-mac-arm
[Finished ⬇️0.26% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.38% ⬆️0.0%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] 13335454 ec2-t3-xlarge-us-east-2
[Finished] 13335454 test-mac-arm
[Finished] 13335454 ursa-i9-9960x
[Finished] 13335454 ursa-thinkcentre-m75q
[Finished] b40fb2c7 ec2-t3-xlarge-us-east-2
[Failed] b40fb2c7 test-mac-arm
[Finished] b40fb2c7 ursa-i9-9960x
[Finished] b40fb2c7 ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java

[ursabot]

['Python', 'R'] benchmarks have high level of regressions.
test-mac-arm