Update virtualenv requirement from ~=20.24.3 to ~=21.6.1

Updates the requirements on [virtualenv](https://github.com/pypa/virtualenv) to permit the latest version.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.24.3...21.6.1)

---
updated-dependencies:
- dependency-name: virtualenv
  dependency-version: 21.6.1
  dependency-type: direct:development
...

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

Author: dependabot[bot]

setup.cfg

@@ -107,7 +107,7 @@ dev =
     types-requests~=2.32.0.20240914
     types-six~=1.16.21.20240513
     types-waitress~=3.0.0.20240423
-    virtualenv~=20.24.3
+    virtualenv~=21.6.1
 
 # 👉 Note: The ``-stubs`` and ``types-`` dependencies above ↑ in the ``dev``
 # extra must be duplicated in ``.pre-commit-config.yaml`` in order for ``tox``