twilio-python [Snyk] Fix for 3 vulnerabilities

This PR was automatically created by Snyk using the credentials of a real user.

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:
- requirements.txt

Vulnerabilities that will be fixed

By pinning:

Issue	Upgrade	Breaking Change	Exploit Maturity
Insufficient Verification of Data Authenticity SNYK-PYTHON-CERTIFI-3164749	`certifi:` `2021.10.8 -> 2023.7.22`	No	No Known Exploit
Improper Following of a Certificate's Chain of Trust SNYK-PYTHON-CERTIFI-5805047	`certifi:` `2021.10.8 -> 2023.7.22`	No	No Known Exploit
Information Exposure SNYK-PYTHON-REQUESTS-5595532	`requests:` `2.27.1 -> 2.31.0`	No	No Known Exploit

Some vulnerabilities couldn't be fully fixed and so Snyk will still find them when the project is tested again. This may be because the vulnerability existed within more than one direct dependency, but not all of the affected dependencies could be upgraded.

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

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