sqlite-export-qgis3.py

options = QgsVectorFileWriter.SaveVectorOptions()
options.driverName = 'SQLite'
options.fileEncoding = 'utf-8'
options.datasourceOptions = ['SPATIALITE=YES',
                             'OGR_SQLITE_PRAGMA=synchronous=OFF,journal_mode=MEMORY,locking_mode=EXCLUSIVE'
                             ]
options.layerOptions = ['SPATIAL_INDEX=NO', 'FID=id']
options.onlySelectedFeatures = True
options.layerName = layer.name()
options.actionOnExistingFile = QgsVectorFileWriter.CreateOrOverwriteLayer


result, msg = QgsVectorFileWriter.writeAsVectorFormat(
    layer,
    output_file,
    options)

if result != QgsVectorFileWriter.NoError:
    print(msg)