fix: fix code to make tests of session and otks databases pass on windows The existing code had a few issues: sessions_dbm.py: detect dumbdbm when used on windows python and properly delete session/otks databases so clear() works. make sure the Session/Otks.cache_db_type() is called even when the database is newly created. test/session_common.py: close session and otks database in teardown before deleting database directory to prevent errors from deleting open files on windows. test/test_sqlite.py: close the session and otks databases opened by SessionTest.setUp(self) so the salite session/otks database are closed before the new anydbm sessions databases are opened for testing the sqlite main db and anydbm as session/otks db. Again this causes deletion of database test directory to fail on windows as you can't delete open files.