Bump tox from 2.9.1 to 3.18.1

Bumps [tox](https://github.com/tox-dev/tox) from 2.9.1 to 3.18.1.
- [Release notes](https://github.com/tox-dev/tox/releases)
- [Changelog](https://github.com/tox-dev/tox/blob/master/docs/changelog.rst)
- [Commits](https://github.com/tox-dev/tox/compare/2.9.1...3.18.1)

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

requirements-indirect.txt

@@ -23,6 +23,6 @@ pyrsistent==0.16.0
 smmap==3.0.4
 smmap2==3.0.1
 toml==0.10.1
-tox==2.9.1
+tox==3.18.1
 virtualenv==20.0.29
 wcwidth==0.1.9