Update extracting-illustrated-pages.md

[anisa-hawes]

I am replacing a broken link https://requests.readthedocs.io/en/master/ at line 78 of Extracting Illustrated Pages from Digital Libraries with Python with https://requests.readthedocs.io/en/latest/user/install/#install.

Closes #2607

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[anisa-hawes]

Hello @svmelton. A very quick Lesson Maintenance task! This PR fixes a broken link identified by @DanielAlvesLABDH in the course of preparing to publish two new PT lessons, #2606 and #2605.