bpo-31177: Skip deleted attributes while calling reset_mock

[tirkarthi]

Skip the deleted attributes while calling reset_mock so that it doesn't cause an AttributeError .

Thanks

https://bugs.python.org/issue31177

[mariocj89]

I'd suggest rewording this to something around:

"Fix bug that prevented using reset_mock on mock instances with deleted attributes"

[tirkarthi]

Thanks @mariocj89 for the review. I have made the suggested changes.

[mariocj89]

Great! Thanks a lot :)

@vstinner, this seems to fix a bug in unittest.mock. When attributes are deleted in a mock they are marked with a mock.sentinel to prevent automatically creating other mocks dynamically, this is done by adding the _deleted sentinel to the child mocks dictionary.
The issue is that _reset_mock iterates over the list of child mocks assuming they are all Mocks without checking for the sentinel _deleted.

[tirkarthi]

@vstinner I have added the below comment to the test. Feel free to suggest rewording if needed.

[bpo-31177](https://bugs.python.org/issue31177): reset_mock should not raise AttributeError when attributes 
were deleted in a mock instance

[vstinner]

LGTM.

[tirkarthi]

Please see #10807 that proposes deleted attributes to be retained that is different from my PR's behavior.

[vstinner]

Oops, I had a draft comment that I forgot to send!

[miss-islington]

Thanks @tirkarthi for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.6.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[miss-islington]

Thanks @tirkarthi for the PR, and @vstinner for merging it 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[bedevere-bot]

GH-10842 is a backport of this pull request to the 3.6 branch.

[bedevere-bot]

GH-10843 is a backport of this pull request to the 3.7 branch.

[vstinner]

Thanks for the fix!