Provide whitelist parity for `MagicMock` and `Mock`

[maxrake]

Description

There are whitelist entries for the return_value and side_effect attributes of the unittest.mock.Mock class. This change seeks to provide parity for the unittest.mock.MagicMock class, which is just a subclass of Mock but with default implementations of most of the magic methods.

Standard library reference: https://docs.python.org/3/library/unittest.mock.html#magic-mock

Checklist:

• [x] I have updated the documentation in the README.md file or my changes don't require an update.
• [x] I have added an entry in CHANGELOG.md.
• [x] ~I have added or adapted tests to cover my changes.~
  • No whitelist tests were found to update
• [x] I have run tox -e fix-style to format my code and checked the result with tox -e style.

[jendrikseipp]

Thanks!