Add missing library 'requests' to requirements.txt

[sammcj]

Without this building the container image fails with:

Creating gpt4all-webui_webui_1 ... done
Attaching to gpt4all-webui_webui_1
webui_1  | Traceback (most recent call last):
webui_1  |   File "/srv/app.py", line 44, in <module>
webui_1  |     import requests
webui_1  | ModuleNotFoundError: No module named 'requests'
gpt4all-webui_webui_1 exited with code 1

Description

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Fixes #24

Type of change

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• [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:

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• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have tested this code locally, and it is working as intended
• [x] I have updated the documentation accordingly

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