Bump virtualenv from 16.7.8 to 20.0.25

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 16.7.8 to 20.0.25.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/master/docs/changelog.rst)
- [Commits](pypa/virtualenv@16.7.8...20.0.25)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -3,5 +3,5 @@ Flask==1.1.1
 itsdangerous==1.1.0
 Jinja2==2.10.3
 MarkupSafe==1.1.1
-virtualenv==16.7.8
+virtualenv==20.0.25
 Werkzeug==0.16.0