Bump virtualenv from 20.25.0 to 20.26.6 (#607)

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.25.0 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@20.25.0...20.26.6)

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

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

Author: dependabot[bot]

requirements.txt

@@ -63,4 +63,4 @@ sphinxcontrib-svg2pdfconverter==1.2.2
 sphinxmark==1.0.0
 typing_extensions==4.9.0
 urllib3==2.2.2
-virtualenv==20.25.0
+virtualenv==20.26.6