Remove unittest's TestCase from top level package import.

[jackton1]

Rationale

When importing from snapshottest package this might result in confusion as the TestCase class name is used by multiple subpackages.

from snapshottest import TestCase  # This results in error for example in a django application.

Solution

Prevent wrong imports by letting the caller specify the unittest package explicitly in the import.

[ayys]

This is good, but I only want to merge it with a major version release since this is a change in the public API.