Bump virtualenv from 16.7.9 to 20.26.6

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 16.7.9 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@16.7.9...20.26.6)

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

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

Author: dependabot[bot]

requirements.txt

@@ -13,7 +13,7 @@ pytz>=2016.10
 sphinx-rtd-theme==0.4.2
 sphinx==1.8.3
 tox==3.14.3
-virtualenv==16.7.9
+virtualenv==20.26.6
 vcrpy==2.0.1
 wheel<=0.39.0
 circuitbreaker>=1.3.1,<2.0.0; python_version <= '3.6'