Update tox requirement from <3.0,>=2.5 to >=2.5,<4.0

Updates the requirements on [tox](https://github.com/tox-dev/tox) to permit the latest version.
- [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@2.5.0...3.14.0)

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

dev-requirements.txt

@@ -4,5 +4,5 @@ pytest>=3.0,<4
 coverage>=4.3,<5
 git+https://github.com/wheerd/sphinx-autodoc-typehints.git@napoleon-master#egg=sphinx-autodoc-napoleon-typehints
 setuptools_scm>=1.15,<2.0
-tox>=2.5,<3.0
+tox>=2.5,<4.0
 pytest-cov>=2.5,<3.0