Update pytest-cov requirement from <3.1,>=3 to >=3,<4.2 in /requirements

Updates the requirements on [pytest-cov](https://github.com/pytest-dev/pytest-cov) to permit the latest version.
- [Changelog](https://github.com/pytest-dev/pytest-cov/blob/master/CHANGELOG.rst)
- [Commits](pytest-dev/pytest-cov@v3.0.0...v4.1.0)

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

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

requirements/testing.txt

@@ -1,6 +1,6 @@
 # Testing dependencies
 # --------------------
 
-pytest-cov>=3,<3.1
+pytest-cov>=3,<4.2
 pytest-qgis>=1.3,<1.4
 semver>=2.13,<2.14