import bpy

objs = bpy.context.selected_objects

# first make sure everything is deselected
bpy.ops.object.select_all(action="DESELECT")

for obj in objs:
    # set the object to be selected and the active object
    obj.select_set(True)
    bpy.context.view_layer.objects.active = obj

    # do the thing here
    bpy.ops.object.convert(target='MESH')

    # then deselect once done
    obj.select_set(False)