use top level imports for tests

tests/test_disk_store.py

@@ -3,7 +3,7 @@
 import typing
 import unittest
 
-from caskdb.disk_store import DiskStorage
+from caskdb import DiskStorage
 
 
 class TempStorageFile:

tests/test_memory_store.py

@@ -1,6 +1,6 @@
 import unittest
 
-from caskdb.memory_store import MemoryStorage
+from caskdb import MemoryStorage
 
 
 class TestInMemoryCaskDB(unittest.TestCase):