Add schema_version on python side, with match test

nancy/db.py

@@ -3,6 +3,9 @@ import os
 import sqlite3
 
 
+schema_version = 2
+
+
 def init(path):
     """
     Initialize a data store directory.

tests/test_db.py

@@ -0,0 +1,8 @@
+def test_schema_version_match():
+    import nancy
+    from nancy.version import schema_version
+
+    cur = nancy._conn.cursor()
+    (db_schema_ver,) = cur.execute("PRAGMA user_version;").fetchone()
+
+    assert schema_version == db_schema_ver