See example failing build

https://github.com/apache/arrow/runs/504483158

test_conversion_extensiontype_to_extensionarray

>   ???
E   pyarrow.lib.ArrowNotImplementedError: No cast implemented from extension<arrow.py_extension_type> to int64

Reporter: Wes McKinney / @wesm
Assignee: Joris Van den Bossche / @jorisvandenbossche

Related issues:

• [Python] Failing test with pandas master for extension type conversion (relates to)

PRs and other links:

• GitHub Pull Request #6614

_{Note: This issue was originally created as ARROW-8106. Please see the migration documentation for further details.}