Feature typeddict alt

[jvanasco]

This is an alternative to #270

Instead of extending test_region as originally requested, the generic backend fixture modified.

The dict comparison is handled by stdlib's unittest comparison function, which historically works well for complex dicts.

[0xDEC0DE]

This fixes #273, but it doesn't prove that it does. In order to do so, you'll need to add config_args to the MemoryBackendTest class that sets cache_dict to a value. That fails when the backend uses dict.pop(), and passes when the backend uses dict.get().

See #274 for some test setup code you can steal.

[jvanasco]

@0xDEC0DE Thanks. I think my test did cover this, just differently than you. I've removed the code, and am closing this ticket in favor of my other one, because your PR was merged.