bpo-45512: Raise exception if sqlite3.Connection.__init__ is called with bad isolatio...

bpo-45512: Raise exception if sqlite3.Connection.__init__ is called with bad isolation level (#29561)

* bpo-45512: Raise sqlite3.Connection.__init__ is called with bad isolation level

* Also explicitly test allowed isolation levels

* Use subTest for better error messages if something goes wrong

* Update Lib/test/test_sqlite3/test_dbapi.py

Co-authored-by: Dong-hee Na <donghee.na92@gmail.com>
Co-authored-by: Dong-hee Na <donghee.na92@gmail.com>