Bump virtualenv from 20.0.12 to 20.0.13

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.0.12 to 20.0.13.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/20.0.13/docs/changelog.rst)
- [Commits](pypa/virtualenv@20.0.12...20.0.13)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

setup.py

@@ -49,7 +49,7 @@
             "python-coveralls==2.9.3",
             "coverage==4.5.4",
             "mock==2.0.0",
-            "virtualenv==20.0.12",
+            "virtualenv==20.0.13",
         ],
     },
     classifiers=[