Ensure slow private attrs are maybe proxies

[mroeschke]

Description

Expected pandas test failures:

tests/indexing/test_indexing.py

Due to this PR, it appears an assert something._values is something_else fails more after this PR since ._values wraps objects in an proxy object now (a known failure mode)

tests/series/indexing/test_setitem.py

Runs into the issue where a test set up calls proxy._values[key] = something using a pandas helper function that isn't proxying correctly

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

One suggestion, otherwise LGTM.

[bdice]

@mroeschke Can you comment on the change in pandas test coverage / pass rate or any metrics like performance of particular code paths that change as a result of this PR?

[mroeschke]

@mroeschke Can you comment on the change in pandas test coverage / pass rate or any metrics like performance of particular code paths that change as a result of this PR?

IIRC the net pandas test pass rate will decrease with this PR (down 1-2%), primarily due to the reasons in #14380 (comment)

However, this PR will help proxy the result of SlowObject._private_method_or_attribute which before would always be a pandas object and not a proxy object.

[bdice]

IIRC the net pandas test pass rate will decrease with this PR (down 1-2%), primarily due to the reasons in #14380 (comment)

Aha! I did not understand that comment until now. Thank you for clarifying what that meant.

[galipremsagar]

/merge