Bump virtualenv from 20.3.1 to 20.4.0 in /requirements (#16354)
Bump virtualenv from 20.3.1 to 20.4.0 in /requirements (#16354)

Bumps [virtualenv](https://github.com/pypa/virtualenv) from 20.3.1 to 20.4.0.
- [Release notes](https://github.com/pypa/virtualenv/releases)
- [Changelog](https://github.com/pypa/virtualenv/blob/main/docs/changelog.rst)
- [Commits](https://github.com/pypa/virtualenv/compare/20.3.1...20.4.0)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>

Co-authored-by: dependabot[bot] <1c358da00a777d4e9898c1280ab801e2df165188@users.noreply.github.com>
mozilla/addons-server,mozilla/olympia,wagnerand/addons-server,bqbn/addons-server,mozilla/addons-server,wagnerand/addons-server,bqbn/addons-server,bqbn/addons-server,diox/olympia,mozilla/addons-server,mozilla/addons-server,mozilla/olympia,mozilla/olympia,bqbn/addons-server,mozilla/olympia,wagnerand/addons-server,diox/olympia,wagnerand/addons-server,diox/olympia,diox/olympia