bpo-45613: Set sqlite3.threadsafety dynamically

[erlend-aasland]

Use the compile-time selected default SQLite threaded mode to set the
DB-API 2.0 attribute 'threadsafety' during sqlite3 module initialisation.

SQLite to DB-API mappings:

• SQLITE_THREADSAFE=0 => threadsafety=0 (single-thread mode)
• SQLITE_THREADSAFE=1 => threadsafety=3 (fully threadsafe)
• SQLITE_THREADSAFE=2 => threadsafety=1 (threadsafe at module level)

https://bugs.python.org/issue45613

[erlend-aasland]

On hold till #29129 is merged. Will rebase and update docs before marking this as ready for review.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @erlend-aasland for commit 396abd4 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[erlend-aasland]

Testing with buildbots to verify that this approach works across different SQLite versions/configurations. I've verified locally that it works with SQLite 3.7.15 (oldest supported), 3.36.0, and current 3.37.0 (dev).

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @erlend-aasland for commit 686d70f 🤖

If you want to schedule another build, you need to add the ":hammer: test-with-buildbots" label again.

[malemburg]

I think the patch is good to go as-is, without the additional changes you mentioned in the comments. Thanks, Erlend.

[erlend-aasland]

I think the patch is good to go as-is, without the additional changes you mentioned in the comments. Thanks, Erlend.

Thanks! There is one change I'd like to make before merging, though: add a versionchanged annotation to the threadsafety attribute doc.

[pablogsal]

LGTM

@malemburg do you have anything else you would like here to be addressed?

Edit: ah, I just saw that you approved before. I am landing this then

[erlend-aasland]

Thanks, Marc-Andre & Pablo 🙏🏻