Bump tox from 3.19.0 to 3.20.0

Bumps [tox](https://github.com/tox-dev/tox) from 3.19.0 to 3.20.0.
- [Release notes](https://github.com/tox-dev/tox/releases)
- [Changelog](https://github.com/tox-dev/tox/blob/master/docs/changelog.rst)
- [Commits](tox-dev/tox@3.19.0...3.20.0)

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

Author: dependabot-preview[bot]

poetry.lock

@@ -39,7 +39,7 @@ pytest-flake8 = "^1.0"
 responses = "^0.10.15"
 pytest-runner = "^5.2"
 towncrier = {version = "^19.2", optional = true}
-tox = "^3.19"
+tox = "^3.20"
 sphinx = {version = "^3.1", optional = true}
 sphinx_rtd_theme = {version = "^0.5.0", optional = true}
 pre-commit = {version = "^2.1", optional = true}