BUILD: Update pytest-cov requirement from <5.1,>=4.0.0 to >=4.0.0,<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]>

pyproject.toml

@@ -42,7 +42,7 @@ tests = [
     "matplotlib>=3.5.0,<3.10",
     "mock>=5.1.0,<5.2",
     "pytest>=7.4.0,<8.4",
-    "pytest-cov>=4.0.0,<5.1",
+    "pytest-cov>=4.0.0,<6.1",
     "pytest-xdist>=3.5.0,<3.7",
     "scikit-rf"
 ]