PyMuPDF function xref_length doesn't exist in version 1.18.0

[nighttardis]

Describe the bug
When trying to run a PDF through Strelka with the current "build/python/backend/requiments.txt", PDF always fail with

"pdf": { "elapsed": 0.000511, "flags": [ "pdf_load_error" ], "total": { "extracted": 0, "objects": 0 } },

This is occurring because the function "xref_length" does not exist in PyMuPDF version 1.18.0

Reviewing the change history for the requirements.txt file it looks like the initial deployment of scan_pdf.py included PyMuPDF version 1.18.10, which does steam to have function. It also appears that on Aug 3rd 2021 the version was bumped to 1.18.15 and then downgrade to 1.18.0 instead of 1.18.10.

Steps to reproduce
Steps to reproduce the behavior:

1. Build the backend using the current "build" folder
2. Attempt to submit a PDF.

Expected behavior
Expectation that PDF are properly parsed.

Screenshots
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Release

• Release: [e.g. 1.19.05.16]

Additional context
Add any other context about the problem here.

[phutelmyer]

Thank you @nighttardis for the heads up. I'll likely be updating ScanPDF in the near future and I'll let you know when it's done testing.

[phutelmyer]

Should be fixed in #200