Bump virtualenv from 20.34.0 to 20.36.1 in /backend

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.34.0 to 20.36.1.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.34.0...20.36.1)

---
updated-dependencies:
- dependency-name: virtualenv
  dependency-version: 20.36.1
  dependency-type: indirect
...

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Author: dependabot[bot]