Import UserAgent from werkzeug.user_agent rather than deprecated module

[tw4l]

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Description

Werkzeug deprecated the useragents module in 2.0.x and removed it entirely in 2.1.x in favor of werkzeug.user_agent. This commit modifies the rewriterapp to import UserAgent from user_agent and sets requirements.txt to ensure versions of werkzeug prior to 2.0.x won't be used.

Motivation and Context

Connected to #745

The deprecation of werkzeug.useragents is causing a 500 Internal Server Error in the wayback machine.

Types of changes

• [ ] Replay fix (fixes a replay specific issue)
• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added or updated tests to cover my changes.
• [ ] All new and existing tests passed.

[tw4l]

These changes seem to have caused another issue stemming from the pinned version of markupsafe:

#10 6.660 Installed /usr/local/lib/python3.7/site-packages/defusedxml-0.7.1-py3.7.egg
#10 6.703 error: MarkupSafe 2.0.1 is installed but MarkupSafe>=2.1.1 is required by {'werkzeug'}

@ikreymer if I remove markupsafe<2.1.0 from pywb's requirements file, it builds/runs successfully again, but given that was added fairly recently in https://github.com/webrecorder/pywb/pull/697, maybe you can share a little context or advice if it's better to just pin werkzeug==2.0.3 for now?