vcr/unittest: Simplify file layout

.. and make "from vcr.unittest import VCRTestCase" work again.

docs/usage.rst

@@ -132,7 +132,7 @@ you can hook on ``_get_vcr``, for example:
 
 See
 `the source
-<https://github.com/kevin1024/vcrpy/blob/master/vcr/unittest/testcase.py>`__
+<https://github.com/kevin1024/vcrpy/blob/master/vcr/unittest.py>`__
 for the default implementations of these methods.
 
 If you implement a ``setUp`` method on your test class then make sure to call

tests/unit/test_unittest.py

@@ -3,7 +3,7 @@ from unittest import TextTestRunner, defaultTestLoader
 from unittest.mock import MagicMock
 from urllib.request import urlopen
 
-from ..testcase import VCRTestCase
+from vcr.unittest import VCRTestCase
 
 
 def test_defaults():

vcr/unittest.py

@@ -2,7 +2,7 @@ import inspect
 import os
 import unittest
 
-from .. import VCR
+from .config import VCR
 
 
 class VCRMixin: