[Snyk] Fix for 6 vulnerabilities

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

⚠️ Warning

``` requests 2.24.0 has requirement urllib3!=1.25.0,!=1.25.1,<1.26,>=1.21.1, but you have urllib3 1.26.7. ```

Vulnerabilities that will be fixed

By pinning:

Priority Score (*)	Issue	Upgrade	Breaking Change	Exploit Maturity
509/1000 Why? Has a fix available, CVSS 5.9	Cryptographic Issues SNYK-PYTHON-CRYPTOGRAPHY-1070544	`cryptography:` `3.2.1 -> 3.3.2`	No	No Known Exploit
654/1000 Why? Has a fix available, CVSS 8.8	Deserialization of Untrusted Data SNYK-PYTHON-NETWORKX-1062709	`networkx:` `2.5 -> 2.6`	No	No Known Exploit
479/1000 Why? Has a fix available, CVSS 5.3	Regular Expression Denial of Service (ReDoS) SNYK-PYTHON-NLTK-1053952	`nltk:` `3.5 -> 3.6.4`	No	No Known Exploit
479/1000 Why? Has a fix available, CVSS 5.3	Regular Expression Denial of Service (ReDoS) SNYK-PYTHON-NLTK-1660190	`nltk:` `3.5 -> 3.6.4`	No	No Known Exploit
506/1000 Why? Proof of Concept exploit, Has a fix available, CVSS 3.7	Regular Expression Denial of Service (ReDoS) SNYK-PYTHON-SCIKITLEARN-1079100	`scikit-learn:` `0.23.2 -> 0.24.2`	No	Proof of Concept
479/1000 Why? Has a fix available, CVSS 5.3	Regular Expression Denial of Service (ReDoS) SNYK-PYTHON-URLLIB3-1533435	`urllib3:` `1.25.11 -> 1.26.5`	No	No Known Exploit

(*) Note that the real score may have changed since the PR was raised.

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 effected dependencies could be upgraded.

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

Note: You are seeing this because you or someone else with access to this repository has authorized Snyk to open fix PRs.

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