import bpy

# Delect objects by type
for o in bpy.context.scene.objects:
    if o.type == 'MESH':
        o.select = True
    else:
        o.select = False

# Call the operator only once
bpy.ops.object.delete()

file_loc = 'C:\\Users\\ANTONIO\\Documents\\Blender\\Scripts\\Tiles\\bigstairs.obj'
imported_object = bpy.ops.import_scene.obj(filepath=file_loc)
obj = bpy.context.selected_objects[0] ####<--Fix

bpy.context.scene.objects.active = obj
bpy.ops.object.editmode_toggle()
bpy.ops.mesh.select_all(action='SELECT')
bpy.ops.mesh.remove_doubles()

bpy.ops.mesh.tris_convert_to_quads()

bpy.ops.object.editmode_toggle()

bpy.ops.export_scene.obj(filepath=file_loc)

print('Imported name: ', obj.name)