[Snyk] Security upgrade lxml from 4.4.1 to 4.6.2

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

``` stevedore 3.3.0 has requirement pbr!=2.1.0,>=2.0.0, but you have pbr 1.10.0. pyOpenSSL 20.0.0 has requirement cryptography>=3.2, but you have cryptography 2.8. psqlgraph 3.0.0 has requirement avro~=1.7, but you have avro file-.avro-VERSION.txt. oslo.utils 4.7.0 has requirement pbr!=2.1.0,>=2.0.0, but you have pbr 1.10.0. oslo.serialization 4.0.1 has requirement pbr!=2.1.0,>=2.0.0, but you have pbr 1.10.0. oslo.i18n 5.0.1 has requirement pbr!=2.1.0,>=2.0.0, but you have pbr 1.10.0. graphql-core 2.3.2 has requirement promise<3,>=2.3, but you have promise 2.2.1. gen3datamodel 3.0.1 has requirement jsonschema~=2.5, but you have jsonschema 3.2.0. debtcollector 2.2.0 has requirement pbr!=2.1.0,>=2.0.0, but you have pbr 1.10.0. authutils 3.1.1 has requirement addict~=2.1, but you have addict 0.2.7. authutils 3.1.1 has requirement python-keystoneclient~=1.8, but you have python-keystoneclient 1.6.0. ```

Vulnerabilities that will be fixed

By pinning:

Severity	Priority Score (*)	Issue	Upgrade	Breaking Change	Exploit Maturity
	651/1000 Why? Recently disclosed, Has a fix available, CVSS 7.3	Cross-site Scripting (XSS) SNYK-PYTHON-LXML-1047473	`lxml:` `4.4.1 -> 4.6.2`	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.

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uc-cdis / peregrine