[Python] Builds on master broken by pandas 1.0.2 release

[asfimport]

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.}

[asfimport]

Wes McKinney / @wesm:
I would guess this is also the cause of our nightly pandas-master build failing

[asfimport]

Wes McKinney / @wesm:
cc @jorisvandenbossche

[asfimport]

Wes McKinney / @wesm:
This is the same issue as ARROW-7857 but I'll leave it open until our master build is fixed

[asfimport]

Joris Van den Bossche / @jorisvandenbossche:
Did a quick PR to "fix" the test, can leave the other issue open to ensure we re-enable the test properly when the underlying issue is fixed (for which I don't have time today, this needs fixing of casting extension types to its storage type: ARROW-7858)

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 6614
#6614