Update requirements.txt

[itslonua]

PyMuPDF==1.21.1

PR 类型 PR types

PR 变化内容类型 PR changes

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• [ ] 这个 PR 是提交到dygraph分支或者是一个cherry-pick，否则请先提交到dygarph分支。 This PR is pushed to the dygraph branch or cherry-picked from the dygraph branch. Otherwise, please push your changes to the dygraph branch.
• [ ] 这个PR清楚描述了功能，帮助评审能提升效率。This PR have fully described what it does such that reviewers can speedup.
• [ ] 这个PR已经经过本地测试。This PR can be convered by current tests or already test locally by you.

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[jzhang533]

close this, since PyMuPDF is now optional