Python 3.6.0的sqlite3模块存在一个bug（见issue 29003），无法执行VACUUM语句。
一执行就出现异常：
Traceback (most recent call last):
File "D:\desktop\cannot_vacuum.py", line 25, in <module>
conn.execute('VACUUM')
sqlite3.OperationalError: cannot VACUUM from within a transaction
这个bug也许会在Python 3.6.1中解决。
在Python 3.6.0，要想让程序正常运行，需要在connect时设置isolation_level参数为None，如下：
conn = sqlite3.connect('test.db', isolation_level=None)
或者这样做：
conn = sqlite3.connect('test.db')
conn.isolation_level = None
查看文档，isolation_level=None表示自动commit：
If you want autocommit mode, then set isolation_level to None.
Otherwise leave it at its default, which will result in a plain “BEGIN” statement, or set it to one of SQLite’s supported isolation levels: “DEFERRED”, “IMMEDIATE” or “EXCLUSIVE”.
Changed in version 3.6: sqlite3 used to implicitly commit an open transaction before DDL statements. This is no longer the case.
如果保持isolation_level为默认值，则会在执行每条SQL语句之前，自动加上"BEGIN"语句，从而形成一个事务。
而VACUUM语句是不能在事务中执行的，因此出现了异常。