build(deps): update pytest-cov requirement from <5.1,>=4 to >=4,<6.1

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@v4.0.0...v6.0.0)

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

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

requirements/testing.txt

@@ -6,5 +6,5 @@
 feedparser>=6.0.11,<6.1
 jsonfeed-util>=1.1.2,<2
 mkdocs-material[imaging]>=9
-pytest-cov>=4,<5.1
+pytest-cov>=4,<6.1
 validator-collection>=1.5,<1.6