datasette 1.0a40

Release: datasette 1.0a40

Same security fix as 0.65.5, plus some neat new features and bug fixes:

• Plugins can now launch and manage background tasks using the new datasette.add_background_task() method. Thanks, Alex Garcia.
• I've migrated Datasette to httpx2 for features like the internal datasette.client.get() method.
• A whole lot of bug fixes, many of them stemming from a recent effort to triage issues for a 1.0 stable release.