Update requirements.txt

[CreatorZZY]

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

TL;DR:

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