[issue38093] Update MagicMock __aenter__ and __aexit__ to return AsyncMock's

[Lisa Roach]

Change by Lisa Roach <lisaroach14 at gmail.com>:


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pull_requests: +15885
pull_request: https://github.com/python/cpython/pull/16299

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<https://bugs.python.org/issue38093>
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