[Snyk] Fix for 4 vulnerabilities

Created by: 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:
  • doc/requirements.txt

Vulnerabilities that will be fixed

By pinning:

Severity	Priority Score (*)	Issue	Upgrade	Breaking Change	Exploit Maturity
	589/1000 Why? Has a fix available, CVSS 7.5	Regular Expression Denial of Service (ReDoS) SNYK-PYTHON-PYGMENTS-1086606	pygments: 2.5.2 -> 2.7.4	No	No Known Exploit
	589/1000 Why? Has a fix available, CVSS 7.5	Denial of Service (DoS) SNYK-PYTHON-PYGMENTS-1088505	pygments: 2.5.2 -> 2.7.4	No	No Known Exploit
	646/1000 Why? Proof of Concept exploit, Has a fix available, CVSS 6.5	Cross-site Scripting (XSS) SNYK-PYTHON-SPHINX-570772	sphinx: 1.8.5 -> 3.0.4 Sphinx: 1.8.5 -> 3.0.4	No	Proof of Concept
	636/1000 Why? Proof of Concept exploit, Has a fix available, CVSS 6.3	Cross-site Scripting (XSS) SNYK-PYTHON-SPHINX-570773	sphinx: 1.8.5 -> 3.0.4 Sphinx: 1.8.5 -> 3.0.4	No	Proof of Concept

(*) 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.

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