bugfix: 修复 iam sdk 拷贝到本地使用后执行 iam_makemigrations 报错的问题

[homholueng]

close #41

[uddmorningsun]

@homholueng 如果你注意到 commit -- 077fe89 ，本次 PR 可以理解成 git revert 077fe8934 ，导致 Django 3.2.X 在 import_module(app_name) 会 raise ImproperlyConfigured

(base) root@69faeb96:/tmp/123321/my_project# /tmp/venv/bin/python -u -m django --version
3.2.4
(base) root@69faeb96:/tmp/123321/my_project# /tmp/venv/bin/python -m pip freeze | grep bk-iam
bk-iam==1.1.19
# 基于 1.1.19 patched PR-50 
(base) root@69faeb96:/tmp/123321/my_project# grep "APP_NAME" /tmp/venv/lib/python3.7/site-packages/iam/contrib/iam_migration/constants.py
APP_NAME = "iam_migration"

(base) root@69faeb96:/tmp/123321/my_project# /tmp/venv/bin/python -u manage.py iam_makemigrations -v3 initial.json
Traceback (most recent call last):
  File "/tmp/venv/lib/python3.7/site-packages/django/apps/config.py", line 244, in create
    app_module = import_module(app_name)
... ...
  File "/tmp/venv/lib/python3.7/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/tmp/venv/lib/python3.7/site-packages/django/apps/registry.py", line 91, in populate
    app_config = AppConfig.create(entry)
  File "/tmp/venv/lib/python3.7/site-packages/django/apps/config.py", line 250, in create
    app_config_class.__qualname__,
django.core.exceptions.ImproperlyConfigured: Cannot import 'iam_migration'. Check that 'iam.contrib.iam_migration.apps.IAMMigrationConfig.name' is correct.

Django 3.2.X 开始支持 automatic discovery appconfig：https://docs.djangoproject.com/en/3.2/releases/3.2/#automatic-appconfig-discovery

如果 bk-iam 对 Django 版本有特殊要求（如：不支持 3.2.X），或许更好的方式去增加版本检查，避免给用户造成过多的困扰。

[homholueng]

@uddmorningsun 感谢你的建议，我们会重新处理一下这里的处理逻辑👏🏻