Rewrite MagicMixin Class

[George-Ogden]

The MagicMixin does a lot of things that weren't documented.
It also doesn't override the constructor.

[George-Ogden]

The tests fail because these methods don't exist on the class - they are added to the instance at runtime.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

xarray (https://github.com/pydata/xarray)
+ xarray/tests/test_backends_file_manager.py: note: In function "test_file_manager_mock_write":
+ xarray/tests/test_backends_file_manager.py:39: error: "Callable[[VarArg(Any), KwArg(Any)], MagicMock]" has no attribute "assert_has_calls"  [attr-defined]
+ xarray/tests/test_backends_file_manager.py: note: At top level:

zulip (https://github.com/zulip/zulip)
+ zerver/tests/test_push_notifications.py:2402: error: "Callable[[], Literal[True]]" has no attribute "return_value"  [attr-defined]

aiohttp-devtools (https://github.com/aio-libs/aiohttp-devtools)
+ tests/test_runserver_watch.py:46: error: "Callable[[VarArg(Any), KwArg(Any)], MagicMock]" has no attribute "side_effect"  [attr-defined]

aiohttp (https://github.com/aio-libs/aiohttp)
+ aiohttp/test_utils.py:573:5: error: Cannot assign to a method  [method-assign]
+ aiohttp/test_utils.py:573:5: note: See https://mypy.rtfd.io/en/stable/_refs.html#code-method-assign for more info
+ aiohttp/test_utils.py:574:5: error: Cannot assign to a method  [method-assign]
+ aiohttp/test_utils.py:574:23: error: Incompatible types in assignment (expression has type "Callable[[Any, str, Any], None]", variable has type "Callable[[VarArg(Any), KwArg(Any)], MagicMock]")  [assignment]

[George-Ogden]

Subclassing Any means that this isn't much of an issue