use python's sqlite3 if pysqlite > 2.6

users have reported errors about vacuuming in a transaction, which
appear to be due to changed handling of DDL statements

anki/db.py

@@ -7,6 +7,10 @@ import time
 
 try:
     from pysqlite2 import dbapi2 as sqlite
+    vi = sqlite.version_info
+    if vi[0] > 2 or vi[1] > 6:
+        # latest pysqlite breaks anki
+        raise ImportError()
 except ImportError:
     from sqlite3 import dbapi2 as sqlite