Update requirements.txt

[CreatorZZY]

We found that https://github.com/bcj/AttrDict met some import wrong. We fix it.

TL;DR:

• [x] You're opening this PR against the current release branch
• [x] Works on both Python 2.7 & Python 3.x
• [x] Commits have been squashed and includes the relevant issue number
• [x] Pull request description contains link to relevant issue or detailed notes on changes
  • [x] This must include example code demonstrating your feature or the bug being fixed
• [x] All existing tests pass
• [x] Your bug fix / feature has associated test coverage
• [x] README.md is updated (if relevant)