Savepoint support for sqlite3 backend

[Chris Lamb]

We can enable savepoints on sqlite if the user has set isolation_level=None in their OPTIONS dict (or they have otherwise set the isolation_level to None) on the django.db.connections.connection sqlite instance. The default is '' (empty string).

Patch attached.

[Russell Keith-Magee]

As with #15507 -- if SQLite supports a feature, we should to.

Same comment goes for the patch -- are you sure the feature check is correct? Has this feature always been available? What are you basing your assertion about autocommit upon (I can't find any reference to this limitation from a quick scan of the SQLite docs on savepoints)

See also #14204 -- support for FK constraints in SQLite.

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Andrei Antoukh]

I could not find the official announcement, but according to the documentation of APSW:

Savepoints were introduced in SQLite 3.6.8 (​http://apidoc.apsw.googlecode.com/hg/pysqlite.html#what-apsw-does-better)

I've done my own tests, comments in the code that clarify some things that I found and contrasting with official documentation of sqlite and pysqlite: ​https://gist.github.com/3061376

This case is very similar to #14204, the difference is the minimum version of sqlite.

[Andrei Antoukh]

I think we should go deprecating older versions of sqlite to equal what is being done with support for versions of python. And take advantage of the new things that are incorporated sqlite.

[Claude Paroz]

This was already reported in #13742.