[Snyk] Security upgrade PyYAML from 5.1.2 to 5.2

[xlogix]

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

thinc 7.0.8 requires cymem, which is not installed.
thinc 7.0.8 requires murmurhash, which is not installed.
thinc 7.0.8 requires srsly, which is not installed.
spacy 2.1.9 requires cymem, which is not installed.
spacy 2.1.9 requires murmurhash, which is not installed.
spacy 2.1.9 requires srsly, which is not installed.
preshed 2.0.1 requires cymem, which is not installed.
chatterbot-corpus 1.2.0 has requirement PyYAML<4.0,>=3.12, but you have PyYAML 5.4.1.

Vulnerabilities that will be fixed

By pinning:

Severity	Priority Score (*)	Issue	Upgrade	Breaking Change	Exploit Maturity
	876/1000 Why? Mature exploit, Has a fix available, CVSS 9.8	Improper Access Control SNYK-PYTHON-PYYAML-550022	PyYAML: 5.1.2 -> 5.2	No	Mature
	704/1000 Why? Has a fix available, CVSS 9.8	Arbitrary Code Execution SNYK-PYTHON-PYYAML-559098	PyYAML: 5.1.2 -> 5.2	No	No Known Exploit
	876/1000 Why? Mature exploit, Has a fix available, CVSS 9.8	Arbitrary Code Execution SNYK-PYTHON-PYYAML-590151	PyYAML: 5.1.2 -> 5.2	No	Mature

(*) 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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