[Snyk] Security upgrade urllib3 from 1.25.3 to 1.25.8

[snyk-bot]

Snyk has created this PR to fix one or more vulnerable packages in the `pip` dependencies of this project.

Changes included in this PR

• Changes to the following files to upgrade the vulnerable dependencies to a fixed version:
  • backend/requirements.txt

Vulnerabilities that will be fixed

By pinning:

Severity	Issue	Upgrade	Breaking Change	Exploit Maturity
	Denial of Service (DoS) SNYK-PYTHON-URLLIB3-559452	urllib3: 1.25.3 -> 1.25.8	No	No Known Exploit

Note that some vulnerabilities couldn’t be fully fixed, and so will still fail the Snyk test report.

Check the changes in this PR to ensure they won't cause issues with your project.

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