bpo-38122: minor fixes to AsyncMock spec handling

[voidspace]

Simplification of the spec handling for AsyncMock and correction for how spec_set is handled.

https://bugs.python.org/issue38122

[the-knights-who-say-ni]

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[lisroach]

Thanks for the fix!

[miss-islington]

Thanks @voidspace for the PR, and @matrixise for merging it 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[miss-islington]

Sorry, @voidspace and @matrixise, I could not cleanly backport this to 3.8 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 14fd925a18fe3db0922a7d798e373102fe7a8a9c 3.8

[matrixise]

@voidspace Could you work on the backport for 3.8? Thanks

[bedevere-bot]

GH-16137 is a backport of this pull request to the 3.8 branch.

[matrixise]

@voidspace I have created a PR with the backport, but I would like to get your approval.

[cjw296]

@voidspace: curious as to why these ended up having no tests committed as part of this PR. It's unclear whether https://bugs.python.org/issue38122 is just refactoring or a change in behaviour. If it's the latter, I'd have hoped for some additional tests to verify this...