Update pytest-cov requirement in /requirements

Updates the requirements on [pytest-cov](https://github.com/pytest-dev/pytest-cov) to permit the latest version.
- [Release notes](https://github.com/pytest-dev/pytest-cov/releases)
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v2.11.0...v4.0.0)

---
updated-dependencies:
- dependency-name: pytest-cov
  dependency-type: direct:production
...

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

requirements/testing.txt

@@ -1,5 +1,5 @@
 # Testing dependencies
 # --------------------
 
-pytest-cov>=2.11,<3.1
+pytest-cov>=2.11,<4.1
 semver>=2.13,<2.14