Use sqlean if available in environment (#560)

Closes #559
Closes #235

Refs https://github.com/simonw/llm/issues/60

- Uses `sqlean` in place of `sqlite3` if `sqlean.py` is installed
- Uses `sqlite-dump` if available and `conn.iterdump()` does not exist
- New `with db.ensure_autocommit_off()` method for ensuring autocommit is off, used by `enable_wal()` and `disable_wal()`.

tests/test_cli.py

@@ -1429,7 +1429,7 @@ def test_enable_wal():
             db = Database(dbname)
             db["t"].create({"pk": int}, pk="pk")
             assert db.journal_mode == "delete"
-        result = runner.invoke(cli.cli, ["enable-wal"] + dbs)
+        result = runner.invoke(cli.cli, ["enable-wal"] + dbs, catch_exceptions=False)
         assert 0 == result.exit_code
         for dbname in dbs:
             db = Database(dbname)