build: bump virtualenv version to 21.7.13 to resolve vulns

[georgeRobertson]

TLDR of changes

What kind of changes does this PR introduce?

Tick all that apply

• fix: A bug fix. Correlates with PATCH in SemVer
• feat: A new feature. Correlates with MINOR in SemVer
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
• refactor: A code change that neither fixes a bug nor adds a feature
• perf: A code change that improves performance
• test: Adding missing or correcting existing tests
• build: Changes that affect the build system or external dependencies (example scopes: pip, docker, npm)
• ci: Changes to CI configuration files and scripts (example scopes: GitLabCI)

Please check if the PR fulfills these requirements

• I have read and followed the Contributing guidance
• Docs have been added / updated
• Tests and Linting in the CI are passing
• Changes have been reviewed and approved by a Project Maintainer

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud