Bump virtualenv from 21.2.4 to 21.5.1 (#443)

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 21.2.4 to 21.5.1.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@21.2.4...21.5.1)

---
updated-dependencies:
- dependency-name: virtualenv
  dependency-version: 21.5.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

backend/requirements.txt

@@ -53,5 +53,5 @@ tomlkit==0.15.0
 typing_extensions==4.15.0
 uritemplate==4.2.0
 urllib3==2.7.0
-virtualenv==21.2.4
+virtualenv==21.5.1
 whitenoise==6.12.0