Bump virtualenv from 20.25.1 to 20.26.6

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.25.1 to 20.26.6.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.25.1...20.26.6)

---
updated-dependencies:
- dependency-name: virtualenv
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

requirements_dev.txt

@@ -346,7 +346,7 @@ urllib3==2.2.2
     #   twine
 validate-pyproject[all]==0.16
     # via pyee (pyproject.toml)
-virtualenv==20.25.1
+virtualenv==20.26.6
     # via tox
 watchdog==4.0.0
     # via mkdocs